NAME
    Class::DBI - Simple Database Abstraction

SYNOPSIS
      package Music::DBI;
      use base 'Class::DBI';
      Music::DBI->connection('dbi:mysql:dbname', 'username', 'password');

      package Music::Artist;
      use base 'Music::DBI';
      Music::Artist->table('artist');
      Music::Artist->columns(All => qw/artistid name/);
      Music::Artist->has_many(cds => 'Music::CD');

      package Music::CD;
      use base 'Music::DBI';
      Music::CD->table('cd');
      Music::CD->columns(All => qw/cdid artist title year reldate/);
      Music::CD->has_many(tracks => 'Music::Track');
      Music::CD->has_a(artist => 'Music::Artist');
      Music::CD->has_a(reldate => 'Time::Piece',
        inflate => sub { Time::Piece->strptime(shift, "%Y-%m-%d") },
        deflate => 'ymd',
      );

      Music::CD->might_have(liner_notes => LinerNotes => qw/notes/);

      package Music::Track;
      use base 'Music::DBI';
      Music::Track->table('track');
      Music::Track->columns(All => qw/trackid cd position title/); 

      #-- Meanwhile, in a nearby piece of code! --#

      my $artist = Music::Artist->create({ artistid => 1, name => 'U2' });

      my $cd = $artist->add_to_cds({ 
        cdid   => 1,
        title  => 'October',
        year   => 1980,
      });

      # Oops, got it wrong.
      $cd->year(1981);
      $cd->update;

      # etc.

      foreach my $track ($cd->tracks) {
        print $track->position, $track->title
      }

      $cd->delete; # also deletes the tracks

      my $cd  = Music::CD->retrieve(1);
      my @cds = Music::CD->retrieve_all;
      my @cds = Music::CD->search(year => 1980);
      my @cds = Music::CD->search_like(title => 'October%');

INTRODUCTION
    Class::DBI provides a convenient abstraction layer to a database.

    It not only provides a simple database to object mapping layer, but can
    be used to implement several higher order database functions (triggers,
    referential integrity, cascading delete etc.), at the application level,
    rather than at the database.

    This is particularly useful when using a database which doesn't support
    these (such as MySQL), or when you would like your code to be portable
    across multiple databases which might implement these things in
    different ways.

    In short, Class::DBI aims to make it simple to introduce 'best practice'
    when dealing with data stored in a relational database.

  How to set it up
    *Set up a database.*
        You must have an existing database set up, have DBI.pm installed and
        the necessary DBD:: driver module for that database. See DBI and the
        documentation of your particular database and driver for details.

    *Set up a table for your objects to be stored in.*
        Class::DBI works on a simple one class/one table model. It is your
        responsibility to have your database tables already set up.
        Automating that process is outside the scope of Class::DBI.

        Using our CD example, you might declare a table something like this:

          CREATE TABLE cd (
            cdid   INTEGER   PRIMARY KEY,
            artist INTEGER, # references 'artist'
            title  VARCHAR(255),
            year   CHAR(4),
          );

    *Set up an application base class*
        It's usually wise to set up a "top level" class for your entire
        application to inherit from, rather than have each class inherit
        directly from Class::DBI. This gives you a convenient point to place
        system-wide overrides and enhancements to Class::DBI's behavior.

          package Music::DBI;
          use base 'Class::DBI';

    *Give it a database connection*
        Class::DBI needs to know how to access the database. It does this
        through a DBI connection which you set up by calling the
        connection() method.

          Music::DBI->connection('dbi:mysql:dbname', 'user', 'password');

        By setting the connection up in your application base class all the
        table classes that inherit from it will share the same connection.

    *Set up each Class*
          package Music::CD;
          use base 'Music::DBI';

        Each class will inherit from your application base class, so you
        don't need to repeat the information on how to connect to the
        database.

    *Declare the name of your table*
        Inform Class::DBI what table you are using for this class:

          Music::CD->table('cd');

    *Declare your columns.*
        This is done using the columns() method. In the simplest form, you
        tell it the name of all your columns (with the single primary key
        first):

          Music::CD->columns(All => qw/cdid artist title year/);

        If the primary key of your table spans multiple columns then declare
        them using a separate call to columns() like this:

          Music::CD->columns(Primary => qw/pk1 pk2/);
          Music::CD->columns(Others => qw/foo bar baz/);

        For more information about how you can more efficiently use subsets
        of your columns, see "LAZY POPULATION"

    *Done.*
        That's it! You now have a class with methods to create(),
        retrieve(), search() for, update() and delete() objects from your
        table, as well as accessors and mutators for each of the columns in
        that object (row).

    Let's look at all that in more detail:

CLASS METHODS
  connection
      __PACKAGE__->connection($data_source, $user, $password, \%attr);

    This sets up a database connection with the given information.

    This uses Ima::DBI to set up an inheritable connection (named Main). It
    is therefore usual to only set up a connection() in your application
    base class and let the 'table' classes inherit from it.

      package Music::DBI;
      use base 'Class::DBI';

      Music::DBI->connection('dbi:foo:dbname', 'user', 'password');

      package My::Other::Table;
      use base 'Music::DBI';

    Class::DBI helps you along a bit to set up the database connection.
    connection() provides its own default attributes depending on the driver
    name in the data_source parameter. The connection() method provides
    defaults for these attributes:

      FetchHashKeyName   => 'NAME_lc',
      ShowErrorStatement => 1,
      ChopBlanks         => 1,
      AutoCommit         => 1,

    (Except for Oracle and Pg, where AutoCommit defaults 0, placing the
    database in transactional mode).

    The defaults can always be extended (or overridden if you know what
    you're doing) by supplying your own \%attr parameter. For example:

      Music::DBI->connection(dbi:foo:dbname','user','pass',{ChopBlanks=>0});

    The RootClass of DBIx::ContextualFetch in also inherited from Ima::DBI,
    and you should be very careful not to change this unless you know what
    you're doing!

   Dynamic Database Connections / db_Main
    It is sometimes desirable to generate your database connection
    information dynamically, for example, to allow multiple databases with
    the same schema to not have to duplicate an entire class hierarchy.

    The preferred method for doing this is to supply your own db_Main()
    method rather than calling connection(). This method should return a
    valid database handle, and should ensure it sets the standard attributes
    described above, preferably by combining $class->_default_attributes()
    with your own. Note, this handle *must* have its RootClass set to
    DBIx::ContextualFetch, so it is usually not possible to just supply a
    $dbh obtained elsewhere.

    Note that connection information is class data, and that changing it at
    run time may have unexpected behaviour for instances of the class
    already in existence.

  table
      __PACKAGE__->table($table);

      $table = Class->table;
      $table = $obj->table;

    An accessor to get/set the name of the database table in which this
    class is stored. It -must- be set.

    Table information is inherited by subclasses, but can be overridden.

  table_alias
      package Shop::Order;
      __PACKAGE__->table('orders');
      __PACKAGE__->table_alias('orders');

    When Class::DBI constructs SQL, it aliases your table name to a name
    representing your class. However, if your class's name is an SQL
    reserved word (such as 'Order') this will cause SQL errors. In such
    cases you should supply your own alias for your table name (which can,
    of course, be the same as the actual table name).

    This can also be passed as a second argument to 'table':

      __PACKAGE__-->table('orders', 'orders');

    As with table, this is inherited but can be overridden.

  sequence / auto_increment
      __PACKAGE__->sequence($sequence_name);

      $sequence_name = Class->sequence;
      $sequence_name = $obj->sequence;

    If you are using a database which supports sequences and you want to use
    a sequence to automatically supply values for the primary key of a
    table, then you should declare this using the sequence() method:

      __PACKAGE__->columns(Primary => 'id');
      __PACKAGE__->sequence('class_id_seq');

    Class::DBI will use the sequence to generate a primary key value when
    objects are created without one.

    *NOTE* This method does not work for Oracle. However, Class::DBI::Oracle
    (which can be downloaded separately from CPAN) provides a suitable
    replacement sequence() method.

    If you are using a database with AUTO_INCREMENT (e.g. MySQL) then you do
    not need this, and any call to create() without a primary key specified
    will fill this in automagically.

    Sequence and auto-increment mechanisms only apply to tables that have a
    single column primary key. For tables with multi-column primary keys you
    need to supply the key values manually.

CONSTRUCTORS and DESTRUCTORS
    The following are methods provided for convenience to create, retrieve
    and delete stored objects. It's not entirely one-size fits all and you
    might find it necessary to override them.

  create
      my $obj = Class->create(\%data);

    This is a constructor to create a new object and store it in the
    database.

    %data consists of the initial information to place in your object and
    the database. The keys of %data match up with the columns of your
    objects and the values are the initial settings of those fields.

      my $cd = Music::CD->create({ 
        cdid   => 1,
        artist => $artist,
        title  => 'October',
        year   => 1980,
      });

    If the table has a single primary key column and that column value is
    not defined in %data, create() will assume it is to be generated. If a
    sequence() has been specified for this Class, it will use that.
    Otherwise, it will assume the primary key can be generated by
    AUTO_INCREMENT and attempt to use that.

    The "before_create" trigger is invoked directly after storing the
    supplied values into the new object and before inserting the record into
    the database. The object stored in $self may not have all the
    functionality of the final object after_creation, particularly if the
    database is going to be providing the primary key value.

    For tables with multi-column primary keys you need to supply all the key
    values, either in the arguments to the create() method, or by setting
    the values in a "before_create" trigger.

    If the class has declared relationships with foreign classes via
    has_a(), you can pass an object to create() for the value of that key.
    Class::DBI will Do The Right Thing.

    After the new record has been inserted into the database the data for
    non-primary key columns is discarded from the object. If those columns
    are accessed again they'll simply be fetched as needed. This ensures
    that the data in the application is consistent with what the database
    *actually* stored.

    The "after_create" trigger is invoked after the database insert has
    executed.

  find_or_create
      my $cd = Music::CD->find_or_create({ artist => 'U2', title => 'Boy' });

    This checks if a CD can be found to match the information passed, and if
    not creates it.

  delete
      $obj->delete;
      Music::CD->search(year => 1980, title => 'Greatest %')->delete_all;

    Deletes this object from the database and from memory. If you have set
    up any relationships using "has_many" or "might_have", this will delete
    the foreign elements also, recursively (cascading delete). $obj is no
    longer usable after this call.

    Multiple objects can be deleted by calling delete_all on the Iterator
    returned from a search. Each object found will be deleted in turn, so
    cascading delete and other triggers will be honoured.

    The "before_delete" trigger is when an object instance is about to be
    deleted. It is invoked before any cascaded deletes. The "after_delete"
    trigger is invoked after the record has been deleted from the database
    and just before the contents in memory are discarded.

RETRIEVING OBJECTS
    Class::DBI provides a few very simple search methods.

    It is not the goal of Class::DBI to replace the need for using SQL.
    Users are expected to write their own searches for more complex cases.

    Class::DBI::AbstractSearch, available on CPAN, provides a much more
    complex search interface than Class::DBI provides itself.

  retrieve
      $obj = Class->retrieve( $id );
      $obj = Class->retrieve( %key_values );

    Given key values it will retrieve the object with that key from the
    database. For tables with a single column primary key a single parameter
    can be used, otherwise a hash of key-name key-value pairs must be given.

      my $cd = Music::CD->retrieve(1) or die "No such cd";

  retrieve_all
      my @objs = Class->retrieve_all;
      my $iterator = Class->retrieve_all;

    Retrieves objects for all rows in the database. This is probably a bad
    idea if your table is big, unless you use the iterator version.

  search
      @objs = Class->search(column1 => $value, column2 => $value ...);

    This is a simple search for all objects where the columns specified are
    equal to the values specified e.g.:

      @cds = Music::CD->search(year => 1990);
      @cds = Music::CD->search(title => "Greatest Hits", year => 1990);

    You may also specify the sort order of the results by adding a final
    hash of arguments with the key 'order_by':

      @cds = Music::CD->search(year => 1990, { order_by=>'artist' });

  search_like
      @objs = Class->search_like(column1 => $like_pattern, ....);

    This is a simple search for all objects where the columns specified are
    like the values specified. $like_pattern is a pattern given in SQL LIKE
    predicate syntax. '%' means "any one or more characters", '_' means "any
    single character".

      @cds = Music::CD->search_like(title => 'October%');
      @cds = Music::CD->search_like(title => 'Hits%', artist => 'Various%');

    You can also use 'order_by' with these, as with search().

ITERATORS
      my $it = Music::CD->search_like(title => 'October%');
      while (my $cd = $it->next) {
        print $cd->title;
      }

    Any of the above searches (as well as those defined by has_many) can
    also be used as an iterator. Rather than creating a list of objects
    matching your criteria, this will return a Class::DBI::Iterator
    instance, which can return the objects required one at a time.

    Currently the iterator initially fetches all the matching row data into
    memory, and defers only the creation of the objects from that data until
    the iterator is asked for the next object. So using an iterator will
    only save significant memory if your objects will inflate substantially
    when used.

    In the case of has_many relationships with a mapping method, the mapping
    method is not called until each time you call 'next'. This means that if
    your mapping is not a one-to-one, the results will probably not be what
    you expect.

  Subclassing the Iterator
      Music::CD->iterator_class('Music::CD::Iterator');

    You can also subclass the default iterator class to override its
    functionality. This is done via class data, and so is inherited into
    your subclasses.

  QUICK RETRIEVAL
      my $obj = Class->construct(\%data);

    This is used to turn data from the database into objects, and should
    thus only be used when writing constructors. It is very handy for
    cheaply setting up lots of objects from data for without going back to
    the database.

    For example, instead of doing one SELECT to get a bunch of IDs and then
    feeding those individually to retrieve() (and thus doing more SELECT
    calls), you can do one SELECT to get the essential data of many objects
    and feed that data to construct():

       return map $class->construct($_), $sth->fetchall_hash;

    The construct() method creates a new empty object, loads in the column
    values, and then invokes the "select" trigger.

COPY AND MOVE
  copy
      $new_obj = $obj->copy;
      $new_obj = $obj->copy($new_id);
      $new_obj = $obj->copy({ title => 'new_title', rating => 18 });

    This creates a copy of the given $obj, removes the primary key, sets any
    supplied column values and calls create() to insert a new record in the
    database.

    For tables with a single column primary key, copy() can be called with
    no parameters and the new object will be assigned a key automatically.
    Or a single parameter can be supplied and will be used as the new key.

    For tables with a multi-column primary key, copy() must be called with
    parameters which supply new values for all primary key columns, unless a
    "before_create" trigger will supply them. The create() method will fail
    if any primary key columns are not defined.

      my $blrunner_dc = $blrunner->copy("Bladerunner: Director's Cut");
      my $blrunner_unrated = $blrunner->copy({
        Title => "Bladerunner: Director's Cut",
        Rating => 'Unrated',
      });

  move
      my $new_obj = Sub::Class->move($old_obj);
      my $new_obj = Sub::Class->move($old_obj, $new_id);
      my $new_obj = Sub::Class->move($old_obj, \%changes);

    For transferring objects from one class to another. Similar to copy(),
    an instance of Sub::Class is created using the data in $old_obj
    (Sub::Class is a subclass of $old_obj's subclass). Like copy(), you can
    supply $new_id as the primary key of $new_obj (otherwise the usual
    sequence or autoincrement is used), or a hashref of multiple new values.

TRIGGERS
      __PACKAGE__->add_trigger(trigger_point_name => \&code_to_execute);

      # e.g.

      __PACKAGE__->add_trigger(after_create  => \&call_after_create);

    It is possible to set up triggers that will be called at various points
    in the life of an object. Valid trigger points are:

      before_create       (also used for deflation)
      after_create
      before_set_$column  (also used by add_constraint)
      after_set_$column   (also used for inflation and by has_a)
      before_update       (also used for deflation and by might_have)
      after_update
      before_delete
      after_delete
      select              (also used for inflation and by construct and _flesh)

    You can create any number of triggers for each point, but you cannot
    specify the order in which they will be run.

    All triggers are passed the object they are being fired for, except when
    "before_set_$column" is fired during create, in which case the class is
    passed in place of the object, which does not yet exist. You may change
    object values if required.

    Some triggers are also passed extra parameters as name-value pairs. The
    individual triggers are further documented with the methods that trigger
    them.

CONSTRAINTS
      __PACKAGE__->add_constraint('name', column => \&check_sub);

      # e.g.

      __PACKAGE__->add_constraint('over18', age => \&check_age);

      # Simple version
      sub check_age { 
        my ($value) = @_;
        return $value >= 18;
      }

      # Cross-field checking - must have SSN if age < 18
      sub check_age { 
        my ($value, $self, $column_name, $changing) = @_;
        return 1 if $value >= 18;     # We're old enough. 
        return 1 if $changing->{SSN}; # We're also being given an SSN
        return 0 if !ref($self);      # This is a create, so we can't have an SSN
        return 1 if $self->ssn;       # We already have one in the database
        return 0;                     # We can't find an SSN anywhere
      }

    It is also possible to set up constraints on the values that can be set
    on a column. The constraint on a column is triggered whenever an object
    is created and whenever the value in that column is being changed.

    The constraint code is called with four parameters:

      - The new value to be assigned
      - The object it will be assigned to
      (or class name when initially creating an object)
      - The name of the column
      (useful if many constraints share the same code)
      - A hash ref of all new column values being assigned
      (useful for cross-field validation)

    The constraints are applied to all the columns being set before the
    object data is changed. Attempting to create or modify an object where
    one or more constraint fail results in an exception and the object
    remains unchanged.

    Note 1: Constraints are implemented using before_set_$column triggers.
    This will only prevent you from setting these values through a the
    provided create() or set() methods. It will always be possible to bypass
    this if you try hard enough.

    Note 2: When an object is created constraints are currently only checked
    for column names included in the parameters to create(). This is
    probably a bug and is likely to change in future.

  constrain_column
      Film->constrain_column(year => qr/^\d{4}$/);
      Film->constrain_column(rating => [qw/U Uc PG 12 15 18/]);

    Simple anonymous constraints can also be added to a column using the
    constrain_column() method. By default this takes either a regex which
    must match, or a reference to a list of possible values.

    However, this behaviour can be extended (or replaced) by providing a
    constraint handler for the type of argument passed to constrain_column.
    This behavior should be provided in a method named
    "_constrain_by_$type", where $type is the moniker of the argument. For
    example, the two shown above would be provided by _constrain_by_array()
    and _constrain_by_regexp().

DATA NORMALIZATION
    Before an object is assigned data from the application (via create or a
    set accessor) the normalize_column_values() method is called with a
    reference to a hash containing the column names and the new values which
    are to be assigned (after any validation and constraint checking, as
    described below).

    Currently Class::DBI does not offer any per-column mechanism here. The
    default method is empty. You can override it in your own classes to
    normalize (edit) the data in any way you need. For example the values in
    the hash for certain columns could be made lowercase.

    The method is called as an instance method when the values of an
    existing object are being changed, and as a class method when a new
    object is being created.

DATA VALIDATION
    Before an object is assigned data from the application (via create or a
    set accessor) the validate_column_values() method is called with a
    reference to a hash containing the column names and the new values which
    are to be assigned.

    The method is called as an instance method when the values of an
    existing object are being changed, and as a class method when a new
    object is being created.

    The default method calls the before_set_$column trigger for each column
    name in the hash. Each trigger is called inside an eval. Any failures
    result in an exception after all have been checked. The exception data
    is a reference to a hash which holds the column name and error text for
    each trigger error.

    When using this mechanism for form data validation, for example, this
    exception data can be stored in an exception object, via a custom
    _croak() method, and then caught and used to redisplay the form with
    error messages next to each field which failed validation.

EXCEPTIONS
    All errors that are generated, or caught and propagated, by Class::DBI
    are handled by calling the _croak() method (as an instance method if
    possible, or else as a class method).

    The _croak() method is passed an error message and in some cases some
    extra information as described below. The default behaviour is simply to
    call Carp::croak($message).

    Applications that require custom behaviour should override the _croak()
    method in their application base class (or table classes for
    table-specific behaviour). For example:

      use Error;

      sub _croak {
        my ($self, $message, %info) = @_;
        # convert errors into exception objects
        # except for duplicate insert errors which we'll ignore
        Error->throw(-text => $message, %info)
          unless $message =~ /^Can't insert .* duplicate/;
        return;
      }

    The _croak() method is expected to trigger an exception and not return.
    If it does return then it should use "return;" so that an undef or empty
    list is returned as required depending on the calling context. You
    should only return other values if you are prepared to deal with the
    (unsupported) consequences.

    For exceptions that are caught and propagated by Class::DBI, $message
    includes the text of $@ and the original $@ value is available in
    $info{err}. That allows you to correctly propagate exception objects
    that may have been thrown 'below' Class::DBI (using
    Exception::Class::DBI for example).

    Exceptions generated by some methods may provide additional data in
    $info{data} and, if so, also store the method name in $info{method}. For
    example, the validate_column_values() method stores details of failed
    validations in $info{data}. See individual method documentation for what
    additional data they may store, if any.

WARNINGS
    All warnings are handled by calling the _carp() method (as an instance
    method if possible, or else as a class method). The default behaviour is
    simply to call Carp::carp().

INSTANCE METHODS
  accessors
    Class::DBI inherits from Class::Accessor and thus provides individual
    accessor methods for every column in your subclass. It also overrides
    the get() and set() methods provided by Accessor to automagically handle
    database reading and writing. (Note that as it doesn't make sense to
    store a list of values in a column, set() takes a hash of column =>
    value pairs, rather than the single key => values of Class::Accessor).

  the fundamental set() and get() methods
      $value = $obj->get($column_name);
      @values = $obj->get(@column_names);

      $obj->set($column_name => $value);
      $obj->set($col1 => $value1, $col2 => $value2 ... );

    These methods are the fundamental entry points for getting and setting
    column values. The extra accessor methods automatically generated for
    each column of your table are simple wrappers that call these get() and
    set() methods.

    The set() method calls normalize_column_values() then
    validate_column_values() before storing the values. The
    "before_set_$column" trigger is invoked by validate_column_values(),
    checking any constraints that may have been set up.

    The "after_set_$column" trigger is invoked after the new value has been
    stored.

    It is possible for an object to not have all its column data in memory
    (due to lazy inflation). If the get() method is called for such a column
    then it will select the corresponding group of columns and then invoke
    the "select" trigger.

Changing Your Column Accessor Method Names
  accessor_name / mutator_name
    If you want to change the name of your accessors, you need to provide an
    accessor_name() method, which will convert a column name to a method
    name.

    e.g: if your local naming convention was to prepend the word 'customer'
    to each column in the 'customer' table, so that you had the columns
    'customerid', 'customername' and 'customerage', you would end up with
    code filled with calls to $customer->customerid,
    $customer->customername, $customer->customerage etc. By creating an
    accessor_name method like:

      sub accessor_name {
        my ($class, $column) = @_;
        $column =~ s/^customer//;
        return $column;
      }

    Your methods would now be the simpler $customer->id, $customer->name and
    $customer->age etc.

    Similarly, if you want to have distinct accessor and mutator methods,
    you would provide a mutator_name() method which would return the name of
    the method to change the value:

      sub mutator_name {
        my ($class, $column) = @_;
        return "set_$column";
      }

    If you override the mutator_name, then the accessor method will be
    enforced as read-only, and the mutator as write-only.

  update vs auto update
    There are two modes for the accessors to work in: manual update and
    autoupdate. When in autoupdate mode, every time one calls an accessor to
    make a change an UPDATE will immediately be sent to the database.
    Otherwise, if autoupdate is off, no changes will be written until
    update() is explicitly called.

    This is an example of manual updating:

      # The calls to NumExplodingSheep() and Rating() will only make the
      # changes in memory, not in the database.  Once update() is called
      # it writes to the database in one swell foop.
      $gone->NumExplodingSheep(5);
      $gone->Rating('NC-17');
      $gone->update;

    And of autoupdating:

      # Turn autoupdating on for this object.
      $gone->autoupdate(1);

      # Each accessor call causes the new value to immediately be written.
      $gone->NumExplodingSheep(5);
      $gone->Rating('NC-17');

    Manual updating is probably more efficient than autoupdating and it
    provides the extra safety of a discard_changes() option to clear out all
    unsaved changes. Autoupdating can be more convenient for the programmer.
    Autoupdating is *off* by default.

    If changes are neither updated nor rolled back when the object is
    destroyed (falls out of scope or the program ends) then Class::DBI's
    DESTROY method will print a warning about unsaved changes.

  autoupdate
      __PACKAGE__->autoupdate($on_or_off);
      $update_style = Class->autoupdate;

      $obj->autoupdate($on_or_off);
      $update_style = $obj->autoupdate;

    This is an accessor to the current style of auto-updating. When called
    with no arguments it returns the current auto-updating state, true for
    on, false for off. When given an argument it turns auto-updating on and
    off: a true value turns it on, a false one off.

    When called as a class method it will control the updating style for
    every instance of the class. When called on an individual object it will
    control updating for just that object, overriding the choice for the
    class.

      __PACKAGE__->autoupdate(1);     # Autoupdate is now on for the class.

      $obj = Class->retrieve('Aliens Cut My Hair');
      $obj->autoupdate(0);      # Shut off autoupdating for this object.

    The update setting for an object is not stored in the database.

  update
      $obj->update;

    If "autoupdate" is not enabled then changes you make to your object are
    not reflected in the database until you call update(). It is harmless to
    call update() if there are no changes to be saved. (If autoupdate is on
    there'll never be anything to save.)

    Note: If you have transactions turned on for your database (but see
    "TRANSACTIONS" below) you will also need to call dbi_commit(), as
    update() merely issues the UPDATE to the database).

    After the database update has been executed, the data for columns that
    have been updated are deleted from the object. If those columns are
    accessed again they'll simply be fetched as needed. This ensures that
    the data in the application is consistent with what the database
    *actually* stored.

    When update() is called the "before_update"($self) trigger is always
    invoked immediately.

    If any columns have been updated then the "after_update" trigger is
    invoked after the database update has executed and is passed: ($self,
    discard_columns => \@discard_columns)

    The trigger code can modify the discard_columns array to affect which
    columns are discarded.

    For example:

      Class->add_trigger(after_update => sub {
        my ($self, %args) = @_;
        my $discard_columns = $args{discard_columns};
        # discard the md5_hash column if any field starting with 'foo'
        # has been updated - because the md5_hash will have been changed
        # by a trigger.
        push @$discard_columns, 'md5_hash' if grep { /^foo/ } @$discard_columns;
      });

    Take care to not delete a primary key column unless you know what you're
    doing.

    The update() method returns the number of rows updated. If the object
    had not changed and thus did not need to issue an UPDATE statement, the
    update() call will have a return value of -1.

    If the record in the database has been deleted, or its primary key value
    changed, then the update will not affect any records and so the update()
    method will return 0.

  discard_changes
      $obj->discard_changes;

    Removes any changes you've made to this object since the last update.
    Currently this simply discards the column values from the object.

    If you're using autoupdate this method will throw an exception.

  is_changed
      my $changed = $obj->is_changed;
      my @changed_keys = $obj->is_changed;

    Indicates if the given $obj has changes since the last update. Returns a
    list of keys which have changed. (If autoupdate is on, this method will
    return an empty list, unless called inside a before_update or
    after_set_$column trigger)

  id
      $id = $obj->id;
      @id = $obj->id;

    Returns a unique identifier for this object based on the values in the
    database. It's the equivalent of $obj->get($self->columns('Primary')),
    with inflated values reduced to their ids.

    A warning will be generated if this method is used in scalar context on
    a table with a multi-column primary key.

  LOW-LEVEL DATA ACCESS
    On some occasions, such as when you're writing triggers or constraint
    routines, you'll want to manipulate data in a Class::DBI object without
    using the usual get() and set() accessors, which may themselves call
    triggers, fetch information from the database, etc.

    Rather than interacting directly with the data hash stored in a
    Class::DBI object (the exact implementation of which may change in
    future releases) you could use Class::DBI's low-level accessors. These
    appear 'private' to make you think carefully about using them - they
    should not be a common means of dealing with the object.

    The data within the object is modelled as a set of key-value pairs,
    where the keys are normalized column names (returned by find_column()),
    and the values are the data from the database row represented by the
    object. Access is via these functions:

    _attrs
          @values = $object->_attrs(@cols);

        Returns the values for one or more keys.

    _attribute_store
          $object->_attribute_store( { $col0 => $val0, $col1 => $val1 } );
          $object->_attribute_store($col0, $val0, $col1, $val1);

        Stores values in the object. They key-value pairs may be passed in
        either as a simple list or as a hash reference. This only updates
        values in the object itself; changes will not be propagated to the
        database.

    _attribute_set
          $object->_attribute_set( { $col0 => $val0, $col1 => $val1 } );
          $object->_attribute_set($col0, $val0, $col1, $val1);

        Updates values in the object via _attribute_store(), but also logs
        the changes so that they are propagated to the database with the
        next update. (Unlike set(), however, _attribute_set() will not
        trigger an update if autoupdate is turned on.)

    _attribute_delete
          @values = $object->_attribute_delete(@cols);

        Deletes values from the object, and returns the deleted values.

    _attribute_exists
          $bool = $object->_attribute_exists($col);

        Returns a true value if the object contains a value for the
        specified column, and a false value otherwise.

    By default, Class::DBI uses simple hash references to store object data,
    but all access is via these routines, so if you want to implement a
    different data model, just override these functions.

  OVERLOADED OPERATORS
    Class::DBI and its subclasses overload the perl builtin *stringify* and
    *bool* operators. This is a significant convenience.

    The perl builtin *bool* operator is overloaded so that a Class::DBI
    object reference is true so long as all its key columns have defined
    values. (This means an object with an id() of zero is not considered
    false.)

    When a Class::DBI object reference is used in a string context it will,
    by default, return the value of the primary key. (Composite primary key
    values will be separated by a slash).

    You can also specify the column(s) to be used for stringification via
    the special 'Stringify' column group. So, for example, if you're using
    an auto-incremented primary key, you could use this to provide a more
    meaningful display string:

      Widget->columns(Stringify => qw/name/);

    If you need to do anything more complex, you can provide an
    stringify_self() method which stringification will call:

      sub stringify_self { 
        my $self = shift;
        return join ":", $self->id, $self->name;
      }

    This overloading behaviour can be useful for columns that have has_a()
    relationships. For example, consider a table that has price and currency
    fields:

      package Widget;
      use base 'My::Class::DBI';
      Widget->table('widget');
      Widget->columns(All => qw/widgetid name price currency_code/);

      $obj = Widget->retrieve($id);
      print $obj->price . " " . $obj->currency_code;

    The would print something like ""42.07 USD"". If the currency_code field
    is later changed to be a foreign key to a new currency table then
    $obj->currency_code will return an object reference instead of a plain
    string. Without overloading the stringify operator the example would now
    print something like ""42.07 Widget=HASH(0x1275}"" and the fix would be
    to change the code to add a call to id():

      print $obj->price . " " . $obj->currency_code->id;

    However, with overloaded stringification, the original code continues to
    work as before, with no code changes needed.

    This makes it much simpler and safer to add relationships to existing
    applications, or remove them later.

TABLE RELATIONSHIPS
    Databases are all about relationships. Thus Class::DBI provides a way
    for you to set up descriptions of your relationhips.

    Class::DBI provides three such relationships: 'has_a', 'has_many', and
    'might_have'. Others are available from CPAN.

  has_a
      Music::CD->has_a(column => 'Foreign::Class');

      Music::CD->has_a(artist => 'Music::Artist');
      print $cd->artist->name;

    'has_a' is most commonly used to supply lookup information for a foreign
    key. If a column is declared as storing the primary key of another
    table, then calling the method for that column does not return the id,
    but instead the relevant object from that foreign class.

    It is also possible to use has_a to inflate the column value to a non
    Class::DBI based. A common usage would be to inflate a date field to a
    date/time object:

      Music::CD->has_a(reldate => 'Date::Simple');
      print $cd->reldate->format("%d %b, %Y");

      Music::CD->has_a(reldate => 'Time::Piece',
        inflate => sub { Time::Piece->strptime(shift, "%Y-%m-%d") },
        deflate => 'ymd',
      );
      print $cd->reldate->strftime("%d %b, %Y");

    If the foreign class is another Class::DBI representation retrieve is
    called on that class with the column value. Any other object will be
    instantiated either by calling new($value) or using the given 'inflate'
    method. If the inflate method name is a subref, it will be executed, and
    will be passed the value and the Class::DBI object as arguments.

    When the object is being written to the database the object will be
    deflated either by calling the 'deflate' method (if given), or by
    attempting to stringify the object. If the deflate method is a subref,
    it will be passed the Class::DBI object as an argument.

    *NOTE* You should not attempt to make your primary key column inflate
    using has_a() as bad things will happen. If you have two tables which
    share a primary key, consider using might_have() instead.

  has_many
      Class->has_many(method_to_create => "Foreign::Class");

      Music::CD->has_many(tracks => 'Music::Track');

      my @tracks = $cd->tracks;

      my $track6 = $cd->add_to_tracks({ 
        position => 6,
        title    => 'Tomorrow',
      });

    This method declares that another table is referencing us (i.e. storing
    our primary key in its table).

    It creates a named accessor method in our class which returns a list of
    all the matching Foreign::Class objects.

    In addition it creates another method which allows a new associated
    object to be constructed, taking care of the linking automatically. This
    method is the same as the accessor method with "add_to_" prepended.

    The add_to_tracks example above is exactly equivalent to:

      my $track6 = Music::Track->create({
        cd       => $cd,
        position => 6,
        title    => 'Tomorrow',
      });

    When setting up the relationship the foreign class's has_a()
    declarations are examined to discover which of its columns reference our
    class. (Note that because this happens at compile time, if the foreign
    class is defined in the same file, the class with the has_a() must be
    defined earlier than the class with the has_many(). If the classes are
    in different files, Class::DBI should usually be able to do the right
    things, as long as all classes inherit Class::DBI before 'use'ing any
    other classes.)

    If the foreign class has no has_a() declarations linking to this class,
    it is assumed that the foreign key in that class is named after the
    moniker() of this class.

    If this is not true you can pass an additional third argument to the
    has_many() declaration stating which column of the foreign class is the
    foreign key to this class.

   Limiting
      Music::Artist->has_many(cds => 'Music::CD');
      my @cds = $artist->cds(year => 1980);

    When calling the method created by has_many, you can also supply any
    additional key/value pairs for restricting the search. The above example
    will only return the CDs with a year of 1980.

   Ordering
      Music::CD->has_many(tracks => 'Music::Track', { order_by => 'playorder' });

    has_many takes an optional final hashref of options. If an 'order_by'
    option is set, its value will be set in an ORDER BY clause in the SQL
    issued. This is passed through 'as is', enabling order_by clauses such
    as 'length DESC, position'.

   Mapping
      Music::CD->has_many(styles => [ 'Music::StyleRef' => 'style' ]);

    If the second argument to has_many is turned into a listref of the
    Classname and an additional method, then that method will be called in
    turn on each of the objects being returned.

    The above is exactly equivalent to:

      Music::CD->has_many(_style_refs => 'Music::StyleRef');

      sub styles { 
        my $self = shift;
        return map $_->style, $self->_style_refs;
      }

    For an example of where this is useful see "MANY TO MANY RELATIONSHIPS"
    below.

  might_have
      Music::CD->might_have(method_name => Class => (@fields_to_import));

      Music::CD->might_have(liner_notes => LinerNotes => qw/notes/);

      my $liner_notes_object = $cd->liner_notes;
      my $notes = $cd->notes; # equivalent to $cd->liner_notes->notes;

    might_have() is similar to has_many() for relationships that can have at
    most one associated objects. For example, if you have a CD database to
    which you want to add liner notes information, you might not want to add
    a 'liner_notes' column to your main CD table even though there is no
    multiplicity of relationship involved (each CD has at most one 'liner
    notes' field). So, you create another table with the same primary key as
    this one, with which you can cross-reference.

    But you don't want to have to keep writing methods to turn the the
    'list' of liner_notes objects you'd get back from has_many into the
    single object you'd need. So, might_have() does this work for you. It
    creates an accessor to fetch the single object back if it exists, and it
    also allows you import any of its methods into your namespace. So, in
    the example above, the LinerNotes class can be mostly invisible - you
    can just call $cd->notes and it will call the notes method on the
    correct LinerNotes object transparently for you.

    Making sure you don't have namespace clashes is up to you, as is
    correctly creating the objects, but this may be made simpler in later
    versions. (Particularly if someone asks for this!)

  Notes
    has_a(), might_have() and has_many() check that the relevant class has
    already been loaded. If it hasn't then they try to load the module of
    the same name using require. If the require fails because it can't find
    the module then it will assume it's not a simple require (i.e.,
    Foreign::Class isn't in Foreign/Class.pm) and that you will take care of
    it and ignore the warning. Any other error, such as a syntax error,
    triggers an exception.

    NOTE: The two classes in a relationship do not have to be in the same
    database, on the same machine, or even in the same type of database! It
    is quite acceptable for a table in a MySQL database to be connected to a
    different table in an Oracle database, and for cascading delete etc to
    work across these. This should assist greatly if you need to migrate a
    database gradually.

MANY TO MANY RELATIONSHIPS
    Class::DBI does not currently support Many to Many relationships, per
    se. However, by combining the relationships that already exist it is
    possible to set these up.

    Consider the case of Films and Actors, with a linking Role table with a
    multi-column Primary Key. First of all set up the Role class:

      Role->table('role');
      Role->columns(Primary => qw/film actor/);
      Role->has_a(film => 'Film');
      Role->has_a(actor => 'Actor');

    Then, set up the Film and Actor classes to use this linking table:

      Film->table('film');
      Film->columns(All => qw/id title rating/);
      Film->has_many(stars => [ Role => 'actor' ]);

      Actor->table('actor');
      Actor->columns(All => qw/id name/);
      Actor->has_many(films => [ Role => 'film' ]);

    In each case the 'mapping method' variation of has_many() is used to
    call the lookup method on the Role object returned. As these methods are
    the 'has_a' relationships on the Role, these will return the actual
    Actor and Film objects, providing a cheap many-to-many relationship.

    In the case of Film, this is equivalent to the more long-winded:

      Film->has_many(roles => "Role");

      sub actors { 
        my $self = shift;
        return map $_->actor, $self->roles 
      }

    As this is almost exactly what is created internally, add_to_stars and
    add_to_films will generally do the right thing as they are actually
    doing the equivalent of add_to_roles:

      $film->add_to_actors({ actor => $actor });

    Similarly a cascading delete will also do the right thing as it will
    only delete the relationship from the linking table.

    If the Role table were to contain extra information, such as the name of
    the character played, then you would usually need to skip these
    short-cuts and set up each of the relationships, and associated helper
    methods, manually.

ADDING NEW RELATIONSHIP TYPES
  add_relationship_type
    The relationships described above are implemented through
    Class::DBI::Relationship subclasses. These are then plugged into
    Class::DBI through an add_relationship_type() call:

      __PACKAGE__->add_relationship_type(
        has_a      => "Class::DBI::Relationship::HasA",
        has_many   => "Class::DBI::Relationship::HasMany",
        might_have => "Class::DBI::Relationship::MightHave",
      );

    If is thus possible to add new relationship types, or modify the
    behaviour of the existing types. See Class::DBI::Relationship for more
    information on what is required.

DEFINING SQL STATEMENTS
    There are several main approaches to setting up your own SQL queries:

    For queries which could be used to create a list of matching objects you
    can create a constructor method associated with this SQL and let
    Class::DBI do the work for you, or just inline the entire query.

    For more complex queries you need to fall back on the underlying
    Ima::DBI query mechanism. (Caveat: since Ima::DBI uses sprintf-style
    interpolation, you need to be careful to double any "wildcard" % signs
    in your queries).

  add_constructor
      __PACKAGE__->add_constructor(method_name => 'SQL_where_clause');

    The SQL can be of arbitrary complexity and will be turned into:

      SELECT (essential columns)
        FROM (table name)
       WHERE <your SQL>

    This will then create a method of the name you specify, which returns a
    list of objects as with any built in query.

    For example:

      Music::CD->add_constructor(new_music => 'year > 2000');
      my @recent = Music::CD->new_music;

    You can also supply placeholders in your SQL, which must then be
    specified at query time:

      Music::CD->add_constructor(new_music => 'year > ?');
      my @recent = Music::CD->new_music(2000);

  retrieve_from_sql
    On occasions where you want to execute arbitrary SQL, but don't want to
    go to the trouble of setting up a constructor method, you can inline the
    entire WHERE clause, and just get the objects back directly:

      my @cds = Music::CD->retrieve_from_sql(qq{
        artist = 'Ozzy Osbourne' AND
        title like "%Crazy"      AND
        year <= 1986
        ORDER BY year
        LIMIT 2,3
      });

  Ima::DBI queries
    When you can't use 'add_constructor', e.g. when using aggregate
    functions, you can fall back on the fact that Class::DBI inherits from
    Ima::DBI and prefers to use its style of dealing with statements, via
    set_sql().

    The Class::DBI set_sql() method defaults to using prepare_cached()
    unless the $cache parameter is defined and false (see Ima::DBI docs for
    more information).

    To assist with writing SQL that is inheritable into subclasses, several
    additional substitutions are available here: __TABLE__, __ESSENTIAL__
    and __IDENTIFIER__. These represent the table name associated with the
    class, its essential columns, and the primary key of the current object,
    in the case of an instance method on it.

    For example, the SQL for the internal 'update' method is implemented as:

      __PACKAGE__->set_sql('update', <<"");
        UPDATE __TABLE__
        SET    %s
        WHERE  __IDENTIFIER__

    The 'longhand' version of the new_music constructor shown above would
    similarly be:

      Music::CD->set_sql(new_music => qq{
        SELECT __ESSENTIAL__
          FROM __TABLE__
         WHERE year > ?
      });

    For such 'SELECT' queries Ima::DBI's set_sql() method is extended to
    create a helper shortcut method, named by prefixing the name of the SQL
    fragment with 'search_'. Thus, the above call to set_sql() will
    automatically set up the method Music::CD->search_new_music(), which
    will execute this search and return the relevant objects or Iterator.
    (If there are placeholders in the query, you must pass the relevant
    arguments when calling your search method.)

    This does the equivalent of:

      sub search_new_music {
        my ($class, @args) = @_;
        my $sth = $class->sql_new_music;
        $sth->execute(@args);
        return $class->sth_to_objects($sth);
      }

    The $sth which is used to return the objects here is a normal DBI-style
    statement handle, so if the results can't be turned into objects easily,
    it is still possible to call $sth->fetchrow_array etc and return
    whatever data you choose.

    Of course, any query can be added via set_sql, including joins. So, to
    add a query that returns the 10 Artists with the most CDs, you could
    write (with MySQL):

      Music::Artist->set_sql(most_cds => qq{
        SELECT artist.id, COUNT(cd.id) AS cds
          FROM artist, cd
         WHERE artist.id = cd.artist
         GROUP BY artist.id
         ORDER BY cds DESC
         LIMIT 10
      });

      my @artists = Music::Artist->search_most_cds();

    If you also need to access the 'cds' value returned from this query, the
    best approach is to declare 'cds' to be a TEMP column. (See
    "Non-Persistent Fields" below).

  Class::DBI::AbstractSearch
      my @music = Music::CD->search_where(
        artist => [ 'Ozzy', 'Kelly' ],
        status => { '!=', 'outdated' },
      );

    The Class::DBI::AbstractSearch module, available from CPAN, is a plugin
    for Class::DBI that allows you to write arbitrarily complex searches
    using perl data structures, rather than SQL.

  Single Value SELECTs
   select_val
    Selects which only return a single value can couple Class::DBI's
    sql_single() SQL, with the $sth->select_val() call which we get from
    DBIx::ContextualFetch.

      __PACKAGE__->set_sql(count_all => "SELECT COUNT(*) FROM __TABLE__");
      # .. then ..
      my $count = $class->sql_count_all->select_val;

    This can also take placeholders and/or do column interpolation if
    required:

      __PACKAGE__->set_sql(count_above => q{
        SELECT COUNT(*) FROM __TABLE__ WHERE %s > ?
      });
      # .. then ..
      my $count = $class->sql_count_above('year')->select_val(2001);

   sql_single
    Internally Class::DBI defines a very simple SQL fragment called
    'single':

      "SELECT %s FROM __TABLE__".  

    This is used to implement the above Class->count_all():

      $class->sql_single("COUNT(*)")->select_val;

    This interpolates the COUNT(*) into the %s of the SQL, and then executes
    the query, returning a single value.

    Any SQL set up via set_sql() can of course be supplied here, and
    select_val can take arguments for any placeholders there.

    Internally several helper methods are defined using this approach:

    - count_all
    - maximum_value_of($column)
    - minimum_value_of($column)

LAZY POPULATION
    In the tradition of Perl, Class::DBI is lazy about how it loads your
    objects. Often, you find yourself using only a small number of the
    available columns and it would be a waste of memory to load all of them
    just to get at two, especially if you're dealing with large numbers of
    objects simultaneously.

    You should therefore group together your columns by typical usage, as
    fetching one value from a group can also pre-fetch all the others in
    that group for you, for more efficient access.

    So for example, if we usually fetch the artist and title, but don't use
    the 'year' so much, then we could say the following:

      Music::CD->columns(Primary   => qw/cdid/);
      Music::CD->columns(Essential => qw/artist title/);
      Music::CD->columns(Others    => qw/year runlength/);

    Now when you fetch back a CD it will come pre-loaded with the 'cdid',
    'artist' and 'title' fields. Fetching the 'year' will mean another visit
    to the database, but will bring back the 'runlength' whilst it's there.

    This can potentially increase performance.

    If you don't like this behavior, then just add all your non-primary key
    columns to the one group, and Class::DBI will load everything at once.

  columns
      my @all_columns  = $class->columns;
      my @columns      = $class->columns($group);

      my @primary      = $class->primary_columns;
      my $primary      = $class->primary_column;
      my @essential    = $class->_essential;

    There are four 'reserved' groups: 'All', 'Essential', 'Primary' and
    'TEMP'.

    'All' are all columns used by the class. If not set it will be created
    from all the other groups.

    'Primary' is the primary key columns for this class. It *must* be set
    before objects can be used.

    If 'All' is given but not 'Primary' it will assume the first column in
    'All' is the primary key.

    'Essential' are the minimal set of columns needed to load and use the
    object. Only the columns in this group will be loaded when an object is
    retrieve()'d. It is typically used to save memory on a class that has a
    lot of columns but where only use a few of them are commonly used. It
    will automatically be set to 'Primary' if not explicitly set. The
    'Primary' column is always part of the 'Essential' group.

    For simplicity primary_columns(), primary_column(), and _essential()
    methods are provided to return these. The primary_column() method should
    only be used for tables that have a single primary key column.

  Non-Persistent Fields
      Music::CD->columns(TEMP => qw/nonpersistent/);

    If you wish to have fields that act like columns in every other way, but
    that don't actually exist in the database (and thus will not persist),
    you can declare them as part of a column group of 'TEMP'.

  find_column
      Class->find_column($column);
      $obj->find_column($column);

    The columns of a class are stored as Class::DBI::Column objects. This
    method will return you the object for the given column, if it exists.
    This is most useful either in a boolean context to discover if the
    column exists, or to 'normalize' a user-entered column name to an actual
    Column.

    The interface of the Column object itself is still under development, so
    you shouldn't really rely on anything internal to it.

TRANSACTIONS
    Class::DBI suffers from the usual problems when dealing with
    transactions. In particular, you should be very wary when committing
    your changes that you may actually be in a wider scope than expected and
    that your caller may not be expecting you to commit.

    However, as long as you are aware of this, and try to keep the scope of
    your transactions small, ideally always within the scope of a single
    method, you should be able to work with transactions with few problems.

  dbi_commit / dbi_rollback
      $obj->dbi_commit();
      $obj->dbi_rollback();

    These are thin aliases through to the DBI's commit() and rollback()
    commands to commit or rollback all changes to this object.

  Localised Transactions
    A nice idiom for turning on a transaction locally (with AutoCommit
    turned on globally) (courtesy of Dominic Mitchell) is:

      sub do_transaction {
        my $class = shift;
        my ( $code ) = @_;
        # Turn off AutoCommit for this scope.
        # A commit will occur at the exit of this block automatically,
        # when the local AutoCommit goes out of scope.
        local $class->db_Main->{ AutoCommit };

        # Execute the required code inside the transaction.
        eval { $code->() };
        if ( $@ ) {
          my $commit_error = $@;
          eval { $class->dbi_rollback }; # might also die!
          die $commit_error;
        }
      }

      And then you just call:

      Music::DBI->do_transaction( sub {
        my $artist = Music::Artist->create({ name => 'Pink Floyd' });
        my $cd = $artist->add_to_cds({ 
          title => 'Dark Side Of The Moon', 
          year => 1974,
        });
      });

    Now either both will get added, or the entire transaction will be rolled
    back.

UNIQUENESS OF OBJECTS IN MEMORY
    Class::DBI supports uniqueness of objects in memory. In a given perl
    interpreter there will only be one instance of any given object at one
    time. Many variables may reference that object, but there can be only
    one.

    Here's an example to illustrate:

      my $artist1 = Music::Artist->create({ artistid => 7, name => 'Polysics' });
      my $artist2 = Music::Artist->retrieve(7);
      my $artist3 = Music::Artist->search( name => 'Polysics' )->first;

    Now $artist1, $artist2, and $artist3 all point to the same object. If
    you update a property on one of them, all of them will reflect the
    update.

    This is implemented using a simple object lookup index for all live
    objects in memory. It is not a traditional cache - when your objects go
    out of scope, they will be destroyed normally, and a future retrieve
    will instantiate an entirely new object.

    The ability to perform this magic for you replies on your perl having
    access to the Scalar::Util::weaken function. Although this is part of
    the core perl distribution, some vendors do not compile support for it.
    To find out if your perl has support for it, you can run this on the
    command line:

      perl -e 'use Scalar::Util qw(weaken)'

    If you get an error message about weak references not being implemented,
    Class::DBI will not maintain this lookup index, but give you a separate
    instances for each retrieve.

    A few new tools are offered for adjusting the behavior of the object
    index. These are still somewhat experimental and may change in a future
    release.

  remove_from_object_index
      $artist->remove_from_object_index();

    This is an object method for removing a single object from the live
    objects index. You can use this if you want to have multiple distinct
    copies of the same object in memory.

  clear_object_index
      Music::DBI->clear_object_index();

    You can call this method on any class or instance of Class::DBI, but the
    effect is universal: it removes all objects from the index.

  purge_object_index_every
      Music::Artist->purge_object_index_every(2000);

    Weak references are not removed from the index when an object goes out
    of scope. This means that over time the index will grow in memory. This
    is really only an issue for long-running environments like mod_perl, but
    every so often dead references are cleaned out to prevent this. By
    default, this happens every 1000 object loads, but you can change that
    default for your class by setting the 'purge_object_index_every' value.

    (Eventually this may handled in the DESTROY method instead.)

    As a final note, keep in mind that you can still have multiple distinct
    copies of an object in memory if you have multiple perl interpreters
    running. CGI, mod_perl, and many other common usage situations run
    multiple interpreters, meaning that each one of them may have an
    instance of an object representing the same data. However, this is no
    worse than it was before, and is entirely normal for database
    applications in multi-process environments.

SUBCLASSING
    The preferred method of interacting with Class::DBI is for you to write
    a subclass for your database connection, with each table-class
    inheriting in turn from it.

    As well as encapsulating the connection information in one place, this
    also allows you to override default behaviour or add additional
    functionality across all of your classes.

    As the innards of Class::DBI are still in flux, you must exercise
    extreme caution in overriding private methods of Class::DBI (those
    starting with an underscore), unless they are explicitly mentioned in
    this documentation as being safe to override. If you find yourself
    needing to do this, then I would suggest that you ask on the mailing
    list about it, and we'll see if we can either come up with a better
    approach, or provide a new means to do whatever you need to do.

CAVEATS
  Multi-Column Foreign Keys are not supported
    You can't currently add a relationship keyed on multiple columns. You
    could, however, write a Relationship plugin to do this, and the world
    would be eternally grateful...

  Don't change or inflate the value of your primary columns
    Altering your primary key column currently causes Bad Things to happen.
    I should really protect against this.

SUPPORTED DATABASES
    Theoretically Class::DBI should work with almost any standard RDBMS. Of
    course, in the real world, we know that that's not true. It is known to
    work with MySQL, PostgreSQL, Oracle and SQLite, each of which have their
    own additional subclass on CPAN that you should explore if you're using
    them:

      L<Class::DBI::mysql>, L<Class::DBI::Pg>, L<Class::DBI::Oracle>,
      L<Class::DBI::SQLite>

    For the most part it's been reported to work with Sybase, although there
    are some issues with multi-case column/table names. Beyond that lies The
    Great Unknown(tm). If you have access to other databases, please give
    this a test run, and let me know the results.

    This is known not to work with DBD::RAM. As a minimum it requires a
    database that supports table aliasing, and a DBI driver that supports
    placeholders.

CURRENT AUTHOR
    Tony Bowden

AUTHOR EMERITUS
    Michael G Schwern

THANKS TO
    Tim Bunce, Tatsuhiko Miyagawa, Perrin Harkins, Alexander Karelas, Barry
    Hoggard, Bart Lateur, Boris Mouzykantskii, Brad Bowman, Brian Parker,
    Casey West, Charles Bailey, Christopher L. Everett Damian Conway, Dan
    Thill, Dave Cash, David Jack Olrik, Dominic Mitchell, Drew Taylor, Drew
    Wilson, Jay Strauss, Jesse Sheidlower, Jonathan Swartz, Marty Pauley,
    Michael Styer, Mike Lambert, Paul Makepeace, Phil Crow, Richard
    Piacentini, Simon Cozens, Simon Wilcox, Thomas Klausner, Tom Renfro, Uri
    Gutman, William McKee, the Class::DBI mailing list, the POOP group, and
    all the others who've helped, but that I've forgetten to mention.

RELEASE PHILOSOPHY
    Class::DBI now uses a three-level versioning system. This release, for
    example, is version 3.0.3

    The general approach to releases will be that users who like a degree of
    stability can hold off on upgrades until the major sub-version increases
    (e.g. 3.1.0). Those who like living more on the cutting edge can keep up
    to date with minor sub-version releases.

    In general the minor-version releases will be for bug fixes and
    refactorings, whereas new functionality will be held-off until major
    sub-version releases.

    Of course, these aren't hard and fast rules, and we'll need to see how
    this all goes.

  Getting changes accepted
    There is an active Class::DBI community, however I am not part of it. I
    am not on the mailing list, and I don't follow the wiki. I also do not
    follow Perl Monks or CPAN reviews or annoCPAN or whatever the tool du
    jour happens to be.

    If you find a problem with Class::DBI, by all means discuss it in any of
    these places, but don't expect anything to happen unless you actually
    tell me about it.

    The preferred method for doing this is via the CPAN RT interface, which
    you can access at http://rt.cpan.org/ or by emailing
    bugs-Class-DBI@rt.cpan.org

    If you email me personally about Class::DBI issues, then I will probably
    bounce them on to there, unless you specifically ask me not to.
    Otherwise I can't keep track of what all needs fixed. (This of course
    means that if you ask me not to send your mail to RT, there's a much
    higher chance that nothing will every happen about your problem).

  Bug Reports
    If you're reporting a bug then it has a much higher chance of getting
    fixed quicker if you can include a failing test case. This should be a
    completely stand-alone test that could be added to the Class::DBI
    distribution. That is, it should use Test::Simple or Test::More, fail
    with the current code, but pass when I fix the problem. If it needs to
    have a working database to show the problem, then this should preferably
    use SQLite, and come with all the code to set this up. The nice people
    on the mailing list will probably help you out if you need assistance
    putting this together.

    You don't need to include code for actually fixing the problem, but of
    course it's often nice if you can. I may choose to fix it in a different
    way, however, so it's often better to ask first whether I'd like a
    patch, particularly before spending a lot of time hacking.

  Patches
    If you are sending patches, then please send either the entire code that
    is being changed or the output of 'diff -Bub'. Please also note what
    version the patch is against. I tend to apply all patches manually, so
    I'm more interested in being able to see what you're doing than in being
    able to apply the patch cleanly. Code formatting isn't an issue, as I
    automagically run perltidy against the source after any changes, so
    please format for clarity.

    Patches have a much better chance of being applied if they are small.
    People often think that it's better for me to get one patch with a bunch
    of fixes. It's not. I'd much rather get 100 small patches that can be
    applied one by one. A change that I can make and release in five minutes
    is always better than one that needs a couple of hours to ponder and
    work through.

    I often reject patches that I don't like. Please don't take it
    personally. I also like time to think about the wider implications of
    changes. Often a *lot* of time. Feel free to remind me about things that
    I may have forgotten about, but as long as they're on rt.cpan.org I will
    get around to them eventually.

  Feature Requests
    Wish-list requests are fine, although you should probably discuss them
    on the mailing list (or equivalent) with others first. There's quite
    often a plugin somewhere that already does what you want.

    In general I am much more open to discussion on how best to provide the
    flexibility for you to make your Cool New Feature(tm) a plugin rather
    than adding it to Class::DBI itself.

    For the most part the core of Class::DBI already has most of the
    functionality that I believe it will ever need (and some more besides,
    that will probably be split off at some point). Most other things are
    much better off as plugins, with a separate life on CPAN or elsewhere
    (and with me nowhere near the critical path). Most of the ongoing work
    on Class::DBI is about making life easier for people to write extensions
    - whether they're local to your own codebase or released for wider
    consumption.

SUPPORT
    Support for Class::DBI is mostly via the mailing list.

    To join the list, or read the archives, visit
    http://lists.digitalcraftsmen.net/mailman/listinfo/classdbi

    There is also a Class::DBI wiki at http://www.class-dbi.com/

    The wiki contains much information that should probably be in these docs
    but isn't yet. (See above if you want to help to rectify this.)

    As mentioned above, I don't follow the list or the wiki, so if you want
    to contact me individually, then you'll have to track me down
    personally.

    There are lots of 3rd party subclasses and plugins available. For a list
    of the ones on CPAN see:
    http://search.cpan.org/search?query=Class%3A%3ADBI&mode=module

    An article on Class::DBI was published on Perl.com a while ago. It's
    slightly out of date , but it's a good introduction:
    http://www.perl.com/pub/a/2002/11/27/classdbi.html

    The wiki has numerous references to other articles, presentations etc.

    http://poop.sourceforge.net/ provides a document comparing a variety of
    different approaches to database persistence, such as Class::DBI,
    Alazabo, Tangram, SPOPS etc.

LICENSE
    This library is free software; you can redistribute it and/or modify it
    under the same terms as Perl itself.

SEE ALSO
    Class::DBI is built on top of Ima::DBI, DBIx::ContextualFetch,
    Class::Accessor and Class::Data::Inheritable. The innards and much of
    the interface are easier to understand if you have an idea of how they
    all work as well.

