NAME

Rose::DB::Object::Metadata::Relationship::ManyToMany - One to many table relationship metadata object.

SYNOPSIS

  use Rose::DB::Object::Metadata::Relationship::ManyToMany;

  $rel = Rose::DB::Object::Metadata::Relationship::ManyToMany->new(...);
  $rel->make_methods(...);
  ...

DESCRIPTION

Objects of this class store and manipulate metadata for relationships in which rows from one table are connected to rows in another table through an intermediate table that maps between them.

This class inherits from Rose::DB::Object::Metadata::Relationship. Inherited methods that are not overridden will not be documented a second time here. See the Rose::DB::Object::Metadata::Relationship documentation for more information.

EXAMPLE

Consider the following tables.

    CREATE TABLE widgets
    (
      id    SERIAL PRIMARY KEY,
      name  VARCHAR(255)
    );

    CREATE TABLE colors
    (
      id    SERIAL PRIMARY KEY,
      name  VARCHAR(255)
    );

    CREATE TABLE widget_color_map
    (
      id         SERIAL PRIMARY KEY,
      widget_id  INT NOT NULL REFERENCES widgets (id),
      color_id   INT NOT NULL REFERENCES colors (id),
      UNIQUE(widget_id, color_id)
    );

Given these tables, each widget can have zero or more colors, and each color can be applied to zero or more widgets. This is the type of "many to many" relationship that this class is designed to handle.

In order to do so, each of the three of the tables that participate in the relationship must be fronted by its own Rose::DB::Object-derived class. Let's call those classes Widget, Color, and WidgetColorMap.

The class that maps between the other two classes is called the "map class." In this example, it's WidgetColorMap. The map class must have a foreign key and/or "many to one" relationship pointing to each of the two classes that it maps between.

When it comes to actually creating the three classes that participate in a "many to many" relationship, there's a bit of a "chicken and egg" problem. All these classes need to know about each other more or less "simultaneously," but they must be defined in a serial fashion, and may be loaded in any order by the user.

In order to account for this, method creation may be deferred for any foreign key or relationship that does not yet have all the information it requires to do its job. This should be transparent to the developer, provided that following guidelines are obeyed:

  • The map class should use both of the classes that it maps between.

  • The other two classes should use the map class.

Here's a complete example using the Widget, Color, and WidgetColorMap classes. First, the Widget class which has a "many to many" relationship through which it can retrieve its colors. The Widget class needs to load the map class, WidgetColorMap.

  package Widget;

  use WidgetColorMap; # load map class

  use Rose::DB::Object;
  our @ISA = qw(Rose::DB::Object);

  __PACKAGE__->meta->table('widgets');
  __PACKAGE__->meta->columns
  (
    id   => { type => 'int', primary_key => 1 },
    name => { type => 'varchar', length => 255 },
  );

  # Define "many to many" relationship to get colors
  __PACKAGE__->meta->add_relationship
  (
    colors =>
    {
      type      => 'many to many',
      map_class => 'WidgetColorMap',

      # These are only necessary if the relationship is ambiguous
      #map_from  => 'widget',
      #map_to    => 'color',
    },
  );

  __PACKAGE__->meta->initialize;

  1;

Next, the Color class which has a "many to many" relationship through which it can retrieve all the widgets that have this color. The Color class also needs to load the map class, WidgetColorMap.

  package Color;

  use WidgetColorMap; # load map class

  use Rose::DB::Object;
  our @ISA = qw(Rose::DB::Object);

  __PACKAGE__->meta->table('colors');
  __PACKAGE__->meta->columns
  (
    id   => { type => 'int', primary_key => 1 },
    name => { type => 'varchar', length => 255 },
  );

  # Define "many to many" relationship to get widgets
  __PACKAGE__->meta->add_relationship
  (
    widgets =>
    {
      type      => 'many to many',
      map_class => 'WidgetColorMap',

      # These are only necessary if the relationship is ambiguous
      #map_from  => 'color',
      #map_to    => 'widget',
    },
  );

  __PACKAGE__->meta->initialize;

  1;

Finally, the WidgetColorMap class which must load both of the classes that it maps between (Widget and Color) and must have a foreign key or "many to one" relationship that points to each of them.

  package WidgetColorMap;

  # Load both classes that this class maps between
  use Widget;
  use Color;

  use Rose::DB::Object;
  our @ISA = qw(Rose::DB::Object);

  __PACKAGE__->meta->table('widget_color_map');
  __PACKAGE__->meta->columns
  (
    id        => { type => 'int', primary_key => 1 },
    widget_id => { type => 'int' },
    color_id  => { type => 'int' },
  );

  # Define foreign keys that point to each of the two classes 
  # that this class maps between.
  __PACKAGE__->meta->foreign_keys
  (
    color => 
    {
      class => 'Color',
      key_columns => { color_id => 'id' },
    },

    widget => 
    {
      class => 'Widget',
      key_columns => { widget_id => 'id' },
    },  
  );

  __PACKAGE__->meta->initialize;

  1;

Here's an initial set of data and some examples of the above classes in action. First, the data:

  INSERT INTO widgets (id, name) VALUES (1, 'Sprocket');
  INSERT INTO widgets (id, name) VALUES (2, 'Flange');

  INSERT INTO colors (id, name) VALUES (1, 'Red');
  INSERT INTO colors (id, name) VALUES (2, 'Green');
  INSERT INTO colors (id, name) VALUES (3, 'Blue');

  INSERT INTO widget_color_map (widget_id, color_id) VALUES (1, 1);
  INSERT INTO widget_color_map (widget_id, color_id) VALUES (1, 2);
  INSERT INTO widget_color_map (widget_id, color_id) VALUES (2, 3);

Now the code:

  use Widget;
  use Color;

  $widget = Widget->new(id => 1);
  $widget->load;

  @colors = map { $_->name } $widget->colors; # ('Red', 'Green')

  $color = Color->new(id => 1);
  $color->load;

  @widgets = map { $_->name } $c->widgets; # ('Sprocket')

Phew! It's actually not as complex as it seems. On the other hand, there is something to be said for manually creating the colors() and widgets() methods. If you look to see what's being done on your behalf behind the scenes, it's actually not that complex. Most of the work involves determining which columns of which tables point to which columns in which other tables. You, the programmer, already know this information, so manual "many to many" method definitions are usually straightforward.

For example, here's a custom implementation of the Widget class's colors() method:

  package Widget;
  ...
  use WidgetColorMap;
  use Rose::DB::Object::Manager;
  ...

  sub colors
  {
    my $self = shift;

    my $map_records = 
      Rose::DB::Object::Manager->get_objects(
        object_class => 'WidgetColorMap',
        with_objects => [ 'color' ],
        query =>
        [
          widget_id => $self->id
        ]);

    my @colors = map { $_->color } @$map_records;

    return wantarray ? @colors : \@colors;
  }

You might notice that that's actually about the same amount of typing as what's required to setup all the relationships to take advantage of the automatic method generation. On the other hand, the relationship definitions provide a central location for this information, which might aid in maintenance.

In the end, it's up to you. Which technique makes more sense in terms of initial effort and ongoing ease of maintenance is a question you'll have to answer yourself.

METHOD MAP

get_set

Rose::DB::Object::MakeMethods::Generic, objects_by_map, interface => 'get_set' ...

get_set_now

Rose::DB::Object::MakeMethods::Generic, objects_by_map, interface => 'get_set_now' ...

get_set_on_save

Rose::DB::Object::MakeMethods::Generic, objects_by_map, interface => 'get_set_on_save' ...

add_now

Rose::DB::Object::MakeMethods::Generic, objects_by_map, interface => 'add_now' ...

add_on_save

Rose::DB::Object::MakeMethods::Generic, objects_by_map, interface => 'add_on_save' ...

See the Rose::DB::Object::Metadata::Relationship documentation for an explanation of this method map.

CLASS METHODS

default_auto_method_types [TYPES]

Get or set the default list of auto_method_types. TYPES should be a list of relationship method types. Returns the list of default relationship method types (in list context) or a reference to an array of the default relationship method types (in scalar context). The default list contains "get_set_on_save" and "add_on_save".

OBJECT METHODS

build_method_name_for_type TYPE

Return a method name for the relationship method type TYPE.

For the method types "get_set", "get_set_now", and "get_set_on_save", the relationship's name is returned.

For the method types "add_now" and "add_on_save", the relationship's name prefixed with "add_" is returned.

Otherwise, undef is returned.

manager_class [CLASS]

Get or set the name of the Rose::DB::Object::Manager-derived class that the map_class will use to fetch records. The make_methods method will use Rose::DB::Object::Manager if this value is left undefined.

manager_method [METHOD]

Get or set the name of the manager_class class method to call when fetching records. The make_methods method will use get_objects if this value is left undefined.

manager_args [HASHREF]

Get or set a reference to a hash of name/value arguments to pass to the manager_method when fetching objects. For example, this can be used to enforce a particular sort order for objects fetched via this relationship. Modifying the example above:

  Widget->meta->add_relationship
  (
    colors =>
    {
      type         => 'many to many',
      map_class    => 'WidgetColorMap',
      manager_args => { sort_by => Color->meta->table . '.name' },
    },
  );

This would ensure that a Widget's colors() are listed in alphabetical order. Note that the "name" column is prefixed by the name of the table fronted by the Color class. This is important because several tables may have a column named "name." If this relationship is used to form a JOIN in a query along with one of those tables, then the "name" column will be ambiguous. Adding a table name prefix disambiguates the column name.

Also note that the table name is not hard-coded. Instead, it is fetched from the Rose::DB::Object-derived class that fronts the table. This is more verbose, but is a much better choice than including the literal table name when it comes to long-term maintenance of the code.

See the documentation for Rose::DB::Object::Manager's get_objects method for a full list of valid arguments for use with the manager_args parameter, but remember that you can define your own custom manager_class and thus can also define what kinds of arguments manager_args will accept.

map_class [CLASS]

Get or set the name of the Rose::DB::Object-derived class that fronts the table that maps between the other two tables. This class must have a foreign key and/or "many to one" relationship for each of the two tables that it maps between.

In the example above, the map class is WidgetColorMap.

map_from [NAME]

Get or set the name of the "many to one" relationship or foreign key in map_class that points to the object of the current class. Setting this value is only necessary if the map class has more than one foreign key or "many to one" relationship that points to one of the classes that it maps between.

In the example above, the value of map_from would be "widget" when defining the "many to many" relationship in the Widget class, or "color" when defining the "many to many" relationship in the Color class. Neither of these settings is necessary in the example because the WidgetColorMap class has one foreign key that points to each class, so there is no ambiguity.

map_to [NAME]

Get or set the name of the "many to one" relationship or foreign key in map_class that points to the "foreign" object to be fetched. Setting this value is only necessary if the map class has more than one foreign key or "many to one" relationship that points to one of the classes that it maps between.

In the example above, the value of map_from would be "color" when defining the "many to many" relationship in the Widget class, or "widget" when defining the "many to many" relationship in the Color class. Neither of these settings is necessary in the example because the WidgetColorMap class has one foreign key that points to each class, so there is no ambiguity.

query_args [ARRAYREF]

Get or set a reference to an array of query arguments to add to the query passed to the manager_method when fetching objects.

This can be used to limit the objects fetched via this relationship. For example, modifying the example above:

  Widget->meta->add_relationship
  (
    colors =>
    {
      type       => 'many to many',
      map_class  => 'WidgetColorMap',
      query_args => [ name => { like => '%e%' } ],
    },
  );

This would ensure that a Widget's colors() would be limited to those that contain the letter "e". See the documentation for Rose::DB::Object::Manager's get_objects method for a full list of valid query arguments, but remember that you can define your own custom manager_class and thus can also define what kinds of query arguments it takes.

share_db [BOOL]

Get or set a boolean flag that indicates whether or not all of the classes involved in fetching objects via this relationship (including the objects themselves) will share the same Rose::DB-derived db object. Defaults to true.

type

Returns "many to many".

AUTHOR

John C. Siracusa (siracusa@mindspring.com)

COPYRIGHT

Copyright (c) 2005 by John C. Siracusa. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.