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# Association Basics

This guide is based on guides.rubyonrails.org/association_basics.html

## Why Associations?

Associations exist to simplify code that deals with related rows in separate database tables. Without associations, if you had classes such as:

    class Artist < Sequel::Model
    end

    class Album < Sequel::Model
    end

And you wanted to get all of the albums for a given artist (assuming each album was associated with only one artist):

    Album.filter(:artist_id=>@artist.id).all

Or maybe you want to add an album for a given artist:

    Album.create(:artist_id=>@artist.id, :name=>'RF')

With Associations, you can make the above code simpler, by setting up associations between the two models:

    class Artist < Sequel::Model
      one_to_many :albums
    end

    class Album < Sequel::Model
      many_to_one :artist
    end

Then, the code to retrieve albums related to the artist is simpler:

    @artist.albums

As is the code to add a related album to an artist:

    @artist.add_album(:name=>'RF')

## The Types of Associations

Sequel has four different association types built in:

- many_to_one

- one_to_many

- one_to_one

- many_to_many

### many_to_one

The many_to_one association is used when the table for the current class contains a foreign key that references the primary key in the table for the associated class. It is named because there can be many rows in the current table for each row in the associated table.

    # Database schema:
    #  albums             artists
    #   :id           /--> :id
    #   :artist_id --/     :name
    #   :name

    class Album
      # Uses singular form of associated model name
      many_to_one :artist
    end

### one_to_many

The one_to_many association is used when the table for the associated class contains a foreign key that references the primary key in the table for the current class. It is named because for each row in the current table there can be many rows in the associated table:

    # Database schema:
    #  artists            albums
    #   :id   <----\       :id
    #   :name       \----- :artist_id
    #                      :name

    class Artist
      # Uses plural form of associated model name
      one_to_many :albums
    end

### one_to_one

The one_to_one association can be thought of as a subset of the one_to_many association, but where there can only be either 0 or 1 records in the associated table. It is the least frequently used of the four associations. If you assume each artist cannot be associated with more than one album:

    # Database schema:
    #  artists            albums
    #   :id   <----\       :id
    #   :name       \----- :artist_id
    #                      :name

    class Artist
      # Uses singular form of associated model name
      one_to_one :album
    end

### many_to_many

The many_to_many association allows each row in the current table to be associated to many rows in the associated table, and each row in the associated table to many rows in the current table, by using a join table to associate the two tables. If you assume each artist can have multiple albums and each album can have multiple artists:

    # Database schema:
    #  albums
    #   :id   <----\
    #   :name       \     albums_artists
    #                \---- :album_id
    #  artists       /---- :artist_id
    #   :id   <-----/
    #   :name

    class Artist
      # Uses plural form of associated model name
      many_to_many :albums
    end
    class Album
      many_to_many :artists
    end

### Differences Between many_to_one and one_to_one

If you want to setup a 1-1 relationship between two models, you have to use many_to_one in one model, and one_to_one in the other model. How do you know which to use in which model?

The simplest way to remember is that the model whose table has the foreign key uses many_to_one, and the other model uses one_to_one:

    # Database schema:
    #  artists            albums
    #   :id   <----\       :id
    #   :name       \----- :artist_id
    #                      :name

    class Artist
      one_to_one :album
    end
    class Album
      many_to_one :artist
    end

## Most Common Options

### :key

The :key option must be used if the default column symbol that Sequel would use is not the correct column. For example:

    class Album
      # Assumes :key is :artist_id, based on association name of :artist
      many_to_one :artist
    end
    class Artist
      # Assumes :key is :artist_id, based on class name of Artist
      one_to_many :albums
    end

However, if your schema looks like:

    # Database schema:
    #  artists            albums
    #   :id   <----\       :id
    #   :name       \----- :artistid # Note missing underscore
    #                      :name

Then the default :key option will not be correct. To fix this, you need to specify an explicit :key option:

    class Album
      many_to_one :artist, :key=>:artistid
    end
    class Artist
      one_to_many :albumst, :key=>:artistid
    end

For many_to_many associations, the :left_key and :right_key options can be used to specify the column names in the join table, and the :join_table option can be used to specify the name of the join table:

    # Database schema:
    #  albums
    #   :id   <----\
    #   :name       \     albumsartists
    #                \---- :albumid
    #  artists       /---- :artistid
    #   :id   <-----/
    #   :name

    class Artist
      # Note that :left_key refers to the foreign key pointing to the
      # current table, and :right_key the foreign key pointing to the
      # associated table.
      many_to_many :albums, :left_key=>:artistid, :right_key=>:albumid,
        :join_table=>:albumsartists
    end
    class Album
      many_to_many :artists, :left_key=>:albumid, :right_key=>:artistid,
        :join_table=>:albumsartists
    end

### :class

If the class of the association can not be guessed directly by looking at the association name, you need to specify it via the :class option. For example, if you have two separate foreign keys in the albums table that both point to the artists table, maybe to indicate one artist is the vocalist and one is the composer, you’d have to use the :class option:

    # Database schema:
    #  artists            albums
    #   :id   <----\       :id
    #   :name       \----- :vocalist_id
    #                \---- :composer_id
    #                      :name

    class Album
      many_to_one :vocalist, :class=>:Artist
      many_to_one :composer, :class=>:Artist
    end
    class Artist
      one_to_many :vocalist_albums, :class=>:Album, :key=>:vocalist_id
      one_to_many :composer_albums, :class=>:Album, :key=>:composer_id
    end

## Self-referential Associations

Self-referential associations are easy to handle in Sequel. The simplest example is a tree structure:

    # Database schema:
    #  nodes
    #   :id        <--\
    #   :parent_id ---/
    #   :name

    class Node
      many_to_one :parent, :class=>self
      one_to_many :children :key=>:parent_id, :class=>self
    end

For many_to_many self_referential associations, it’s fairly similar. Here’s an example of a directed graph:

    # Database schema:
    #  nodes              edges
    #   :id   <----------- :successor_id
    #   :name       \----- :predecessor_id

    class Node
      many_to_many :direct_successors, :left_key=>:successor_id,
        :right_key=>:predecessor_id, :join_table=>:edges, :class=>self
      many_to_many :direct_predecessors, :right_key=>:successor_id,
        :left_key=>:predecessor_id, :join_table=>:edges, :class=>self
    end

## Methods Added

When you create an association, it’s going to add instance methods to the class related to the association.

All associations are going to have an instance method added with the same name as the association:

    @artist.albums
    @album.artists

many_to_one and one_to_one associations will also have a setter method added to change the associated object:

    @album.artist = Artist.create(:name=>'YJM')

many_to_many and one_to_many associations will have three methods added:

- add_* to associate an object to the current object

- remove_* to disassociate an object from the current object

- remove_all_* to dissociate all currently associated objects

Examples:

    @artist.add_album(@album)
    @artist.remove_album(@album)
    @artist.remove_all_albums

## Dataset Method

In addition to the above methods, associations also add a dataset method that returns a dataset representing the objects in the associated table:

    @album.artist_id
    # 10
    @album.artist_dataset
    # SELECT * FROM artists WHERE (id = 10)

    @artist.id
    # 20
    @artist.albums_dataset
    # SELECT * FROM albums WHERE (artist_id = 20)

The association dataset is just like any other Sequel dataset, in that it can be further filtered, ordered, etc.:

    @artist.albums_dataset.
     filter(:name.like('A%')).
     order(:copies_sold).
     limit(10)
    # SELECT * FROM albums
    # WHERE ((artist_id = 20) AND (name LIKE 'A%'))
    # ORDER BY copies_sold LIMIT 10

## Caching

Associations are cached after being retrieved:

    @artist.album # Not cached - Database Query
    @artist.album # Cached - No Database Query

    @album.artists # Not cached - Database Query
    @album.artists # Cached - No Database Query

You can choose to ignore the cached versions and do a database query to retrieve results by passing a true argument to the association method:

    @album.artists # Not cached - Database Query
    @album.artists # Cached - No Database Query
    @album.artists(true) # Ignore cache - Database Query

If you reload/refresh the object, it will automatically clear the associations cache for the object:

    @album.artists # Not cached - Database Query
    @album.artists # Cached - No Database Query
    @album.reload
    @album.artists # Not Cached - Database Query

If you want direct access to the associations cache, use the associations instance method:

    @album.associations # {}
    @album.associations[:artists] # nil
    @album.artists # [<Artist ...>, ...]
    @album.associations[:artists] # [<Artist ...>, ...]

Note that while the association method caches associated objects, if you retrieve access through the association dataset directly, the results will not be cached:

    @album.artists_dataset.all # [<Artist ...>, ...]
    @album.associations[:artists] # nil

## Name Collisions

Because associations create instance methods, it’s possible to override existing instance methods if you name an extension the same as an existing method. For example, `values` and `associations` would be bad association names.

## Database Schema

Creating an association, doesn’t modify the database schema. Sequel assumes your associations reflect the existing database schema. If not, you should modify your schema before creating the associations.

### many_to_one/one_to_many

For example, for the following model code:

    class Album
      many_to_one :artist
    end
    class Artist
      one_to_many :albums
    end

You probably want the following database schema:

    #  albums             artists
    #   :id           /--> :id
    #   :artist_id --/     :name
    #   :name

Which could be created using the following Sequel code:

    DB.create_table(:artists) do
      # Primary key must be set explicitly
      primary_key :id
      String :name
    end
    DB.create_table(:albums) do
      primary_key :id
      # Table that foreign key references needs to be set explicitly
      # for a database foreign key reference to be created.
      foreign_key :artist_id, :artists
      String :name
    end

If you already had a schema such as:

    # Database schema:
    #  albums             artists
    #   :id                :id
    #   :name              :name

Then you just need to add the column:

    DB.alter_table(:albums) do
      add_foreign_key :artist_id, :artists
    end

### many_to_many

With many_to_many associations, the default join table for the association uses the sorted underscored names of both model classes. For example, with the following model code:

    class Album
      many_to_many :artists
    end
    class Artist
      many_to_many :albums
    end

The default join table name would be `albums_artists`, not `artists_albums`, because:

    ["artists", "albums"].sort.join('_')
    # "albums_artists"

Assume you already had the albums and artists tables created, and you just wanted to add an albums_artists join table to create the following schema:

    # Database schema:
    #  albums
    #   :id   <----\
    #   :name       \     albums_artists
    #                \---- :album_id
    #  artists       /---- :artist_id
    #   :id   <-----/
    #   :name

You could use the following Sequel code:

    DB.create_table(:albums_artists) do
      foreign_key :album_id, :albums
      foreign_key :artist_id, :artists
    end

## Association Scope

If you nest your Sequel::Model classes inside modules, then you should know that Sequel will only look in the same module for associations by default. So the following code will work fine:

    module App
      class Artist < Sequel::Model
        one_to_many :albums
      end
      class Album < Sequel::Model
        many_to_one :artist
      end
    end

However, if you enclose your model classes inside two different modules, things will not work by default:

    module App1
      class Artist < Sequel::Model
        one_to_many :albums
      end
    end
    module App2
      class Album < Sequel::Model
        many_to_one :artist
      end
    end

To fix this, you need to specify the full model class name using the :class option:

    module App1
      class Artist < Sequel::Model
        one_to_many :albums, :class=>"App2::Album"
      end
    end
    module App2
      class Album < Sequel::Model
        many_to_one :artist, :class=>"App1::Artist"
      end
    end

## Method Details

In all of these methods, association is replaced by the symbol you pass to the association.

### association(reload = false) (e.g. albums)

For `many_to_one` and `one_to_one` associations, the association method returns either the single object associated, or nil if no object is associated.

    @artist = @album.artist

For `one_to_many` and `many_to_many` associations, the association method returns an array of associated objects, which may be empty if no objects are currently associated.

    @albums = @artist.albums

### association=(object_to_associate) (e.g. artist=) [`many_to_one` and `one_to_one`]

The association= method sets up an association of the passed object to the current object. For `many_to_one` associations, this sets the foreign key for the current object to point to the associated object’s primary key.

    @album.artist = @artist

For `one_to_one` associations, this sets the foreign key of the associated object to the primary key value of the current object.

For `many_to_one` associations, this does not save the current object. For `one_to_one` associations, this does save the associated object.

### add_association(object_to_associate) (e.g. add_album) [`one_to_many` and `many_to_many`]

The add_association method associates the passed object to the current object. For `one_to_many` associations, it sets the foreign key of the associated object to the primary key value of the current object, and saves the associated object. For `many_to_many` associations, this inserts a row into the join table with the foreign keys set to the primary key values of the current and associated objects. Note that the singular form of the association name is used in this method.

    @artist.add_album(@album)

In addition to passing an actual associated object, you can pass a hash, and a new associated object will be created from them:

    @artist.add_album(:name=>'RF') # creates Album object

The add_association method returns the now associated object:

    @album = @artist.add_album(:name=>'RF')

### remove_association(object_to_disassociate) (e.g. remove_album) [`one_to_many` and `many_to_many`]

The remove_association method disassociates the the passed object from the current object. For `one_to_many` associations, it sets the foreign key of the associated object to NULL, and saves the associated object. For `many_to_many` associations, this deletes the matching row in the join table. Similar to the add_association method, the singular form of the association name is used in this method.

    @artist.remove_album(@album)

Note that this does not delete `@album` from the database, it only disassociates it from the `@artist`. To delete `@album` from the database:

    @album.destroy

The add_association and remove_association methods should be thought of as adding and removing from the association, not from the database.

In addition to passing the object directly to remove_association, you can also pass the associated object’s primary key:

    @artist.remove_album(10)

This will look up the associated object using the key, and remove that album.

The remove_association method returns the now disassociated object:

    @album = @artist.remove_album(10)

### remove_all_association (e.g. remove_all_albums) [`one_to_many` and `many_to_many`]

The remove_all_association method disassociates all currently associated objects. For `one_to_many` associations, it sets the foreign key of all associated objects to NULL in a single query. For `many_to_many` associations, this deletes all matching rows in the join table. Unlike the add_association and remove_association method, the plural form of the association name is used in this method. The remove_all_association method returns the number of rows updated for `one_to_many` associations and the number of rows deleted for `many_to_many` associations:

    @rows_modified = @artist.remove_all_albums

### association_dataset (e.g. albums_dataset)

The association_dataset method returns a dataset that represents all associated objects. This dataset is like any other Sequel dataset, in that it can be filtered, ordered, etc.:

    ds = @artist.albums_dataset.filter(:name.like('A%')).order(:copies_sold)

Unlike most other Sequel datasets, association datasets have a couple of added methods:

    ds.model_object # @artist
    ds.association_reflection # same as Artist.association_reflection(:albums)

For a more info on Sequel’s reflection capabilities see the Reflection page.

## Overriding Method Behavior

Sequel is designed to be very flexible. If the default behavior of the association modification methods isn't what you desire, you can override the methods in your classes. However, you should be aware that for each of the association modification methods described, there is a private method that is preceeded by an underscore that does the actual modification. The public method without the underscore handles caching and callbacks, and shouldn't be overridden by the user.

### _association=

Let’s say you want to set a specific field whenever associating an object using the association setter method. For example, let’s say you have a file_under column for each album to tell you where to file it. If the album is associated with an artist, it should be filed under the artist’s name and the album’s name, otherwise it should just use the album’s name.

    class Album < Sequel::Model
      many_to_one :artist

      private

      def _artist=(artist)
        if artist
          self.artist_id = artist.id
          self.file_under = "#{artist.name}-#{name}"
        else
          self.artist_id = nil
          self.file_under = name
        end
      end
    end

The above example is contrived, as you would generally use a before_save model hook to handle such a modification. However, if you only modify the album’s artist using the artist= method, this approach may perform better.

### _add_association

Continuing with the same example, here’s how would you handle the same case if you also wanted to handle the Artist#add_album method:

    class Artist < Sequel::Model
      one_to_many :albums

      private

      def _add_album(album)
        album.update(:artist_id => id, :file_under=>"#{name}-#{album.name}")
      end
    end

### _remove_association

Continuing with the same example, here’s how would you handle the same case if you also wanted to handle the Artist#remove_album method:

    class Artist < Sequel::Model
      one_to_many :albums

      private

      def _remove_album(album)
        album.update(:artist_id => nil, :file_under=>album.name)
      end
    end

### _remove_all_association

Continuing with the same example, here’s how would you handle the same case if you also wanted to handle the Artist#remove_all_albums method:

    class Artist < Sequel::Model
      one_to_many :albums

      private

      def _remove_all_albums
        # This is Dataset#update, not Model#update, so the :file_under=>:name
        # ends up being "SET file_under = name" in SQL.
        albums_dataset.update(:artist_id => nil, :file_under=>:name)
      end
    end

## Association Options

Sequel’s associations mostly share the same options. For ease of understanding, they are grouped here by section

### Association Dataset Modification Options

#### block

All association defining methods take a block that is passed the default dataset and should return a modified copy of the dataset to use for the association. For example, if you wanted an association that returns all albums of an artist that went gold (sold at least 500,000 copies):

    Artist.one_to_many :gold_albums, :class=>:Album do |ds|
      ds.filter{copies_sold > 500000}
    end

#### :class

This is the class of the associated objects that will be used. It’s one of the most commonly used options. If it is not given, it guesses based on the name of the association. If a *_to_many association is used, uses the singular form of the association name. For example:

    Album.many_to_one :artist # guesses Artist
    Artist.one_to_many :albums # guesses Album

However, for more complex associations, especially ones that add additional filters beyond the foreign/primary key relationships, the default class guessed will be wrong:

    # guesses GoldAlbum
    Artist.one_to_many :gold_albums do |ds|
      ds.filter{copies_sold > 500000}
    end

You can specify the :class option using the class itself, a Symbol, or a String:

    Album.many_to_one :artist, :class=>Artist # Class
    Album.many_to_one :artist, :class=>:Artist # Symbol
    Album.many_to_one :artist, :class=>"Artist" # String

#### :key

For `many_to_one` associations, is the foreign_key in current model’s table that references associated model’s primary key, as a symbol. Defaults to :association_id. Can use an array of symbols for a composite key association.

    Album.many_to_one :artist # :key=>:artist_id

For `one_to_one` and `one_to_many` associations, is the foreign key in associated model’s table that references current model’s primary key, as a symbol. Defaults to :“#{self.name.underscore}_id”.

    Artist.one_to_many :albums # :key=>:artist_id

In both cases an array of symbols for a composite key association:

    Apartment.many_to_one :building # :key=>[:city, :address]

#### :conditions

The conditions to use to filter the association, can be any argument passed to filter. If you use a hash or an array of two element arrays, this will also be used as a filter when using eager_graph to load the association.

    Artist.one_to_many :good_albums, :class=>:Album, :conditions=>{:good=>true}
    @artist.good_albums
    # SELECT * FROM albums WHERE ((artist_id = 1) AND (good IS TRUE))

#### :order

The column(s) by which to order the association dataset. Can be a singular column or an array.

    Artist.one_to_many :albums_by_name, :class=>:Album,
     :order=>:name
    Artist.one_to_many :albums_by_num_tracks, :class=>:Album,
     :order=>[:num_tracks, :name]

#### :select

The columns to SELECT when loading the association. For most associations, it defaults to nil, so * is used. For `many_to_many` associations, it defaults to the associated class’s table_name.*, which means it doesn’t include the columns from the join table. This is to prevent the common issue where the join table includes columns with the same name as columns in the associated table, in which case the joined table’s columns would usually end up clobbering the values in the associated table. If you want to include the join table attributes, you can use this option, but beware that the join table columns can clash with columns from the associated table, so you should alias any columns that have the same name in both the join table and the associated table. Example:

    Artist.one_to_many :albums, :select=>[:id, :name]
    Album.many_to_many :tags, :select=>[:tags.*, :albums_tags__number]

#### :limit

Limit the number of records to the provided value:

    Artist.one_to_many :best_selling_albums, :class=>:Album,
    :order=>:copies_sold, :limit=>5 # LIMIT 5

Use an array with two arguments for the value to specify a limit and an offset.

    Artist.one_to_many :next_best_selling_albums, :class=>:Album,
     :order=>:copies_sold, :limit=>[10, 5] # LIMIT 10 OFFSET 5

This probably doesn’t make a lot of sense for *_to_one associations, though you could use it to specify an offset.

This option is ignored when eager loading.

#### :join_table [`many_to_many`]

Name of table that includes the foreign keys to both the current model and the associated model, as a symbol. Defaults to the name of current model and name of associated model, pluralized, underscored, sorted, and joined with ‘_’. Here’s an example of the defaults:

    Artist.many_to_many :albums # :join_table=>:albums_artists
    Album.many_to_many :artists # :join_table=>:albums_artists
    Person.many_to_many :colleges # :join_table=>:colleges_people

#### :left_key [`many_to_many`]

Foreign key in join table that points to current model’s primary key, as a symbol. Defaults to :“#{self.name.underscore}_id”.

    Album.many_to_many :tags # :left_key=>:album_id

Can use an array of symbols for a composite key association.

#### :right_key [`many_to_many`]

Foreign key in join table that points to associated model’s primary key, as a symbol. Defaults to :“#{name.to_s.singularize}_id”.

    Album.many_to_many :tags # :left_key=>:tag_id

Can use an array of symbols for a composite key association.

#### :distinct

Use the DISTINCT clause when selecting associating object, both when lazy loading and eager loading via eager (but not when using eager_graph).

This is most useful for many_to_many associations that use join tables that contain more than just the foreign keys, where you are storing additional information. For example, if you have a database of people, degree types, and colleges, and you want to return all people from a given college, you may want to use :distinct so that if a person has two separate degrees from the same college, they won’t show up twice.

#### :clone

The :clone option clones an existing association, taking the options you specified for that association, and making a copy of them for this association. Other options provided by this association are then merged into the cloned options.

This is commonly used if you have a bunch of similar associations that you want to DRY up:

    one_to_many :english_verses, :class=>:LyricVerse, :key=>:lyricsongid,
      :order=>:number, :conditions=>{:languageid=>1}
    one_to_many :romaji_verses, :clone=>:english_verses, :conditions=>{:languageid=>2}
    one_to_many :japanese_verses, :clone=>:english_verses, :conditions=>{:languageid=>3}

Note that for the final two asociations, you didn’t have to specify the :class, :key, or :order options, as they were copied by the :clone option. By specifying the :conditions option for the final two associations, it overrides the :conditions option of the first association, it doesn’t attempt to merge them.

In addition to the options hash, the :clone option will copy a block argument from the existing situation. If you want a cloned association to not have the same block as the association you are cloning from, specify the :block=>nil option in additon to the :clone option.

#### :dataset

This is generally only specified for custom associations that aren’t based on primary/foreign key relationships. It should be a proc that is instance evaled to get the base dataset to use before the other options are applied.

Here’s an example of an association of songs to artists through lyrics, where the artist can perform any one of four tasks for the lyric:

    Album.one_to_many :songs, :dataset=>(proc do
      Song.select(:songs.*).
       join(Lyric, :id=>:lyricid,
        id=>[:composer_id, :arranger_id, :vocalist_id, :lyricist_id])
    end)
    Artist.first.songs_dataset
    # SELECT songs.* FROM songs
    # INNER JOIN lyrics ON
    #  lyrics.id = songs.lyric_id AND
    #  1 IN (composer_id, arranger_id, vocalist_id, lyricist_id)

#### :extend

A module or array of modules to extend the dataset with. These are used to set up association extensions. For more information , please see the Advanced Associations page.

#### :primary_key

The column that the :key option references, as a symbol. For `many_to_one` associations, this column in the associated table. For `one_to_one` and `one_to_many` associations, this column in the current table. In both cases, it defaults to the primary key of the table. Can use an array of symbols for a composite key association.

    Artist.set_primary_key :arid
    Artist.one_to_many :albums # :primary_key=>:arid
    Album.one_to_many :artist # :primary_key=>:arid

#### :left_primary_key [`many_to_many`]

Column in current table that :left_key option points to, as a symbol. Defaults to primary key of current table.

    Album.set_primary_key :alid
    Album.many_to_many :tags # :left_primary_key=>:alid

Can use an array of symbols for a composite key association.

#### :right_primary_key [`many_to_many`]

Column in associated table that :right_key points to, as a symbol. Defaults to primary key of the associated table.

    Tag.set_primary_key :tid
    Album.many_to_many :tags # :left_primary_key=>:tid

Can use an array of symbols for a composite key association.

#### :join_table_block [`many_to_many`]

A proc that can be used to modify the dataset used in the add/remove/remove_all methods. It’s separate from the association block, as that is called on a join of the join table and the associated table, whereas this option just applies to the join table. It can be used to make sure additional columns are used when inserting, or that filters are used when deleting.

    Artist.many_to_many :lead_guitar_albums, :join_table_block=>proc do |ds|
      ds.filter(:instrument_id=>5).set_overrides(:instrument_id=>5)
    end

### Callback Options

All callbacks can be specified as a Symbol, Proc, or array of both/either specifying a callback to call. Symbols are interpreted as instance methods that are called with the associated object. Procs are called with the receiver as the first argument and the associated object as the second argument. If an array is given, all of them are called in order.

Before callbacks are often used to check preconditions, they can return false to signal Sequel to abort the modification. If any before callback returns false, the remaining before callbacks are not called and modification is aborted. Before callbacks are also commonly used to modify the current object or the associated object.

After callbacks are often used for notification (logging, email) after a successful modification has been made.

#### :before_add [`one_to_many`, `many_to_many`]

Called before adding an object to the association:

    class Artist
      # Don't allow adding an album to an artist if it has no tracks
      one_to_many :albums, :before_add=>proc{|ar, al| false if al.num_tracks == 0}
    end

#### :after_add [`one_to_many`, `many_to_many`]

Called after adding an object to the association:

    class Artist
      # Log all associations of albums to an audit logging table
      one_to_many :albums, :after_add=>:log_add_album

      private

      def log_add_album(album)
        DB[:audit_logs].insert(:log=>"Album #{album.inspect} associated to #{inspect}")
      end
    end

#### :before_remove [`one_to_many`, `many_to_many`]

Called before removing an object from the association:

    class Artist
      # Don't allow removing a self-titled album
      one_to_many :albums, :before_remove=>proc{|ar, al| false if al.name == ar.name}
    end

#### :after_remove [`one_to_many`, `many_to_many`]

Called after removing an object from the association:

    class Artist
      # Log all disassociations of albums to an audit logging table
      one_to_many :albums, :after_remove=>:log_remove_album

      private

      def log_remove_album(album)
        DB[:audit_logs].insert(:log=>"Album #{album.inspect} disassociated from #{inspect}")
      end
    end

#### :before_set [`many_to_one`, `one_to_one`]

Called before the _association= method is called to modify the objects:

Called before removing an object from the association:

    class Album
      # Don't associate the album with an artist if the year the album was
      # released is less than the year the artist/band started.
      many_to_one :artist, :before_set=>proc{|al, ar| false if al.year < ar.year_started}
    end

#### :after_set [`many_to_one`, `one_to_one`]

Called after the _association= method is called to modify the objects:

    class Album
      # Log all disassociations of albums to an audit logging table
      many_to_one :artist, :after_set=>:log_artist_set

      private

      def log_artist_set(artist)
        DB[:audit_logs].insert(:log=>"Artist for album #{inspect} set to #{artist.inspect}")
      end
    end

#### :after_load

Called after retrieving the associated records from the database. Not called when eager loading via eager_graph, but called when eager loading via eager.

    class Artist
      # Cache all album names to a single string when retrieving the
      # albums.
      one_to_many :albums, :after_add=>:cache_album_names

      attr_reader :album_names

      private

      def cache_album_names(albums)
        @album_names = albums.map{|x| x.name}.join(", ")
      end
    end

Generally used if you know you will always want a certain action done when retrieving the association. However, you need to be careful if you also plan on using eager_graph to eagerly load the association.

For `one_to_many` and `many_to_many` associations, both the argument to symbol callbacks and the second argument to proc callbacks will be an array of associated objects instead of a single object.

#### :uniq [`many_to_many`]

Adds a after_load callback that makes the array of objects unique. In many cases, using the :distinct option is a better approach.

### Eager Loading via eager (query per association) Options

#### :eager

The associations to eagerly load via eager when loading the associated object(s). This is useful for example if you always want to eagerly load dependent associations when loading this association.

For example, if you know that any time that you want to load an artist’s albums, you are also going to want access to the album’s tracks as well:

    # Eager load tracks when loading the albums
    Artist.one_to_many :albums, :eager=>:tracks

You can also use a hash or array to specify multiple dependent associations to eagerly load:

    # Eager load the albums' tracks and the tracks' tags when loading the albums
    Artist.one_to_many :albums, :eager=>{:tracks=>:tags}
    # Eager load the albums' tags and tracks when loading the albums
    Artist.one_to_many :albums, :eager=>[:tags, :tracks]
    # Eager load the albums' tags, tracks, and tracks' tags when loading the albums
    Artist.one_to_many :albums, :eager=>[:tags, {:tracks=>:tags}]

#### :eager_loader

A custom loader to use when eagerly load associated objects via eager. For many details and examples of custom eager loaders, please see the Advanced Associations guide.

#### :eager_loader_key

A symbol for the key column to use to populate the key hash for the eager loader. It can be necessary to specify this for custom eager loaders where the default key that would be used does not exist, as in that case, Sequel would think that the key values are all NULL, and would not attempt to eagerly load any associated objects for that association. If you have a custom eager loader and aren’t sure of a good value to use here, and you aren’t using the key_hash (first argument to the eager_loader proc), then you can probably use the primary key column of the model.

#### :eager_block

If given, should be a proc to use instead of the association method block when eagerly loading. To not use a block when eager loading when one is used normally, should to nil. It’s very uncommon to need this option.

### Eager Loading via eager_graph (one query with joins) Options

#### :eager_graph

The associations to eagerly load via eager_graph when loading the associated object(s). This is useful for example if you always want to eagerly load dependent associations when loading this association, but you want to filter or order the association based on dependent associations:

    Artist.one_to_many :albums_with_short_tracks, :class=>:Album,
     :eager_graph=>:tracks do |ds|
      ds.filter{tracks__seconds < 120}
    end
    Artist.one_to_many :albums_by_track_name, :class=>:Album,
     :eager_graph=>:tracks do |ds|
      ds.order(:tracks__name)
    end

You can also use a hash or array of arguments for :eager_graph, similar to what the :eager option accepts.

#### :graph_conditions

The additional conditions to use on the SQL join when eagerly loading the association via eager_graph. Should be a hash or an array of two element arrays. If not specified, the :conditions option is used if it is a hash or array of two element arrays.

    Artist.one_to_many :active_albums, :class=>:Album,
      :graph_conditions=>{:active=>true}

Note that these conditions on the association are in addition to the default conditions specified by the foreign/primary keys. If you want to replace the conditions specified by the foreign/primary keys, you need the :graph_only_conditions options.

#### :graph_block

The block to pass to Dataset#join_table when eagerly loading the association via eager_graph. This is useful to specify conditions that can’t be specified in a hash or array of two element arrays.

    Artist.one_to_many :gold_albums, :class=>:Album,
      :graph_block=>proc{|j,lj,js| :copies_sold.qualify(j) > 500000}

#### :graph_join_type

The type of SQL join to use when eagerly loading the association via eager_graph. Defaults to :left_outer. This is useful if you want to ensure that all return only artists that have albums:

    Artist.one_to_many :albums, :graph_join_type=>:inner
    # Will exclude artists without an album
    Artist.eager_graph(:albums).all

#### :graph_select

A column or array of columns to select from the associated table when eagerly loading the association via eager_graph. Defaults to all columns in the associated table.

#### :graph_only_conditions

The conditions to use on the SQL join when eagerly loading the association via eager_graph, instead of the default conditions specified by the foreign/primary keys. This option causes the :graph_conditions option to be ignored. This can be useful if the keys you are using are strings and you want to do a case insensitive comparison. For example, let’s say that instead of integer keys, you used string keys based on the album or artist name, and that the album was associated to the artist by name. However, you weren’t enforcing case sensitivity between the keys, so you still want to return albums where the artist’s name differs in case:

    Artist.one_to_many :albums, :key=>:artist_name,
      :graph_only_conditions=>nil,
      :graph_block=>proc{|j,lj,js| {:lower.sql_function(artist_name.qualify(j))=>
        :lower.sql_function(name.qualify(lj))}}

Note how :graph_only_conditions is set to nil to ignore any existing conditions, and :graph_block is used to set up the case insensitive comparison.

Another case where :graph_only_conditions may be used is if you want to use a JOIN USING or NATURAL JOIN for the graph:

    # JOIN USING
    Artist.one_to_many :albums, :key=>:artist_name,
     :graph_only_conditions=>[:artist_name]

    # NATURAL JOIN
    Artist.one_to_many :albums, :key=>:artist_name,
     :graph_only_conditions=>nil, :graph_join_type=>:natural

#### :eager_grapher

Sets up a custom grapher to use when eager loading the objects via eager_graph. This is the eager_graph analogue to the :eager_loader option.

If specified, should be a proc that accepts three three arguments: a dataset, an alias to use for the table to graph for this association, and the alias that was used for the current table (since you can cascade associations). Should return a modified copy of the dataset with the association graphed into it.

    Artist.one_to_many :self_title_albums, :class=>:Album,
     :eager_grapher=>(proc do |ds, ta, iq|
      ds.graph(Album, {:artist_id=>:id, :name=>:name},
        :table_alias=>ta, :implicit_qualifier=>iq)
    end)

This isn’t generally needed, as one of the other eager_graph related association options is usually sufficient.

#### :order_eager_graph

Whether to add the order to the dataset’s order when graphing via eager_graph. Defaults to true, so set to false to disable.

Sequel has to do some guess work when attempting to add the association's order to an eager_graphed dataset. In most cases it does so correctly, but if it has problems, you'll probably want to set this option to false.

#### :graph_join_table_conditions [`many_to_many`]

The additional conditions to use on the SQL join for the join table when eagerly loading the association via eager_graph. Should be a hash or an array of two element arrays.

Let’s say you have a database of people, colleges, and a table called degrees_received that includes a string field specifying the name of the degree, and you want to eager load all colleges for people where the person has received a specific degree:

    Person.many_to_many :bs_degree_colleges, :class=>:College,
      :join_table=>:degrees_received,
      :graph_join_table_conditions=>{:degree=>'BS'}

#### :graph_join_table_block [`many_to_many`]

The block to pass to join_table for the join table when eagerly loading the association via eager_graph. This is used for similar reasons as :graph_block, but is only used for `many_to_many` associations when graphing the join table into the dataset. It’s used in the same place as :graph_join_table_conditions but like :graph_block, is needed for situations where the conditions can’t be specified as a hash or array of two element arrays.

Let’s say you have a database of people, colleges, and a table called degrees_received that includes a string field specifying the name of the degree, and you want to eager load all colleges for people where the person has received a bachelor’s degree (degree starting with B):

    Person.many_to_many :bachelor_degree_colleges, :class=>:College,
      :join_table=>:degrees_received,
      :graph_join_table_block=>proc{|j,lj,js| :degree.qualify(j).like('B%')}

This should be done when graphing the join table, instead of when graphing the final table, as :degree is a column of the join table.

#### :graph_join_table_join_type [`many_to_many`]

The type of SQL join to use for the join table when eagerly loading the association via eager_graph. Defaults to the :graph_join_type option or :left_outer. This exists mainly for consistency in the unlikely case that you want to use a different join type when JOINing to the join table then you want to use for JOINing to the final table

#### :graph_join_table_only_conditions [`many_to_many`]

The conditions to use on the SQL join for the join table when eagerly loading the association via eager_graph, instead of the default conditions specified by the foreign/primary keys. This option causes the :graph_join_table_conditions option to be ignored. This is only useful if you want to replace the default foreign/primary key conditions that Sequel would use when eagerly graphing.

### Advanced Options

#### :reciprocal

The symbol name of the reciprocal association, if it exists. By default, Sequel will try to determine it by looking at the associated model’s assocations for a association that matches the current association’s key(s). Set to nil to not use a reciprocal.

Reciprocals are used in Sequel to modify the matching cached associations in associated objects when calling association methods on the current object. For example, when you retrieve objects in a one_to_many association, it’ll automatically set the matching many_to_one association in the associated objects. The result of this is that code that does this:

    @artist.albums.each{|album| album.artist.name}

only does one database query, because when the @artist’s albums are retrieved, the cached artist association for each album is set to @artist.

In addition to the one_to_many retrieval case, the association modification methods affect the reciprocals as well:

    # Sets the cached artist association for @album to @artist
    @artist.add_album(@album)

    # Sets the cached artist association for @album to nil
    @artist.remove_album(@album)

    # Sets the cached artist association to nil for the @artist's
    # cached albums association
    @artist.remove_all_albums

    # Remove @album from the artist1's cached albums association, and add @album
    # to @artist2's cached albums association.
    @album.artist # @artist1
    @album.artist = @artist2

Sequel can usually guess the correct reciprocal, but if you have multiple associations to the same associated class that use the same keys, you may want to specify the :reciprocal option manually to ensure the correct one is used.

#### :read_only

For `many_to_one` and `one_to_one` associations, do not add a setter method. For `one_to_many` and `many_to_many`, do not add the add_association, remove_association, or remove_all_association methods.

If the default modification methods would not do what you want, and you don’t plan on overriding the internal modification methods to do what you want, it may be best to set this option to true.

#### :validate

Set to false to not validate when implicitly saving any associated object. When using the `one_to_many` association modification methods, the `one_to_one` setter method, or creating a new object by passing a hash to the add_association method, Sequel will automatically save the object. If you don’t want to validate objects when these implicit saves are done, the validate option should be set to false.

#### :allow_eager

If set to false, you cannot load the association eagerly via eager or eager_graph.

    Artist.one_to_many :albums, :allow_eager=>false
    Artist.eager(:albums) # Raises Sequel::Error

This is usually used if the association dataset depends on specific values in model instance that would not be valid when eager loading for multiple instances.

#### :cartesian_product_number

The number of joins completed by this association that could cause more than one row for each row in the current table (default: 0 for *_to_one associations, 1 for *_to_many associations).

This should only be modified in specific cases. For example, if you have a one_to_one association that can actually return more than one row (where the default association method will just return the first), or a many_to_many association where there is a unique index in the join table so that you know only one object will ever be associated through the association.

[Validate]

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