= Advanced Associations

Sequel::Model has the most powerful and flexible associations of any ruby ORM.

== Background: Sequel::Model association options

There are a bunch of advanced association options that are available to
handle more complex cases.  First we'll go over some of
the simpler ones:

All associations take a block that can be used to further filter/modify the
default dataset.  There's also an :eager_block option if you want to use
a different block when eager loading via <tt>Dataset#eager</tt>. Association blocks are
useful for things like:

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

There are a whole bunch of options for changing how the association is eagerly
loaded via <tt>Dataset#eager_graph</tt>: <tt>:graph_block</tt>, <tt>:graph_conditions</tt>,
<tt>:graph_only_conditions</tt>, <tt>:graph_join_type</tt> (and <tt>:graph_join_table_*</tt> ones for
JOINing to the join table in a many_to_many association).

:graph_join_type :: The type of join to do (<tt>:inner</tt>, <tt>:left</tt>, <tt>:right</tt>)
:graph_conditions :: Additional conditions to put on join (needs to be a
                     hash or array of all two pairs).  Automatically assumes unqualified symbols
                     or first element of the pair to be columns of the associated model, and
                     unqualified symbols of the second element of the pair to be columns of the
                     current model.
:graph_block :: A block passed to +join_table+, allowing you to specify
                conditions other than equality, or to use OR, or set up any arbitrary
                condition.  The block is passed the associated table alias, current table 
                alias, and an array of previous joins clause objects.
:graph_only_conditions :: Use these conditions instead of the standard
                          association conditions. This is necessary when you don't want to have an
                          equal condition between the foreign key and primary key of the tables.
                          You can also use this to have a JOIN USING (array of symbols), or a NATURAL
                          or CROSS JOIN (nil, with the appropriate <tt>:graph_join_type</tt>).
  
These can be used like this:

  # Makes Artist.eager_graph(:required_albums).all not return artists that
  # don't have any albums
  Artist.one_to_many :required_albums, :class=>:Album, :graph_join_type=>:inner
  
  # Makes sure all returned albums have the active flag set
  Artist.one_to_many :active_albums, :class=>:Album, \
    :graph_conditions=>{:active=>true}
    
  # Only returns albums that have sold more than 500,000 copies
  Artist.one_to_many :gold_albums, :class=>:Album, \
    :graph_block=>proc{|j,lj,js| :copies_sold.qualify(j) > 500000}
    
  # Handles the case where the tables are associated by a case insensitive name string
  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))}}
    
  # Handles the case where both key columns have the name artist_name, and you want to use
  # a JOIN USING
  Artist.one_to_many :albums, :key=>:artist_name, :graph_only_conditions=>[:artist_name]
  
Remember, using +eager_graph+ is generally only necessary when you need to
filter/order based on columns in an associated table, it is recommended to
use +eager+ for eager loading if possible.

For lazy loading (e.g. Model[1].association), the <tt>:dataset</tt> option can be used
to specify an arbitrary dataset (one that uses different keys, multiple keys,
joins to other tables, etc.).

For eager loading via +eager+, the <tt>:eager_loader</tt> option can be used to specify
how to eagerly load a complex association.  This is an extremely powerful
option.  Though it can often be verbose (compared to other things in Sequel),
it allows you complete control over how to eagerly load associations for a
group of objects.

:eager_loader should be a proc that takes 1 or 3 arguments.  If the proc
takes one argument, it will be given a hash with the following keys:

:key_hash :: A key_hash, described below
:rows :: An array of model objects
:associations :: A hash of dependent associations to eagerly load
:self :: The dataset that is doing the eager loading

If the proc takes three arguments, it gets passed the :key_hash, :rows,
and :associations values.  The only way to get the :self value is to
accept one argument.  The 3 argument procs are allowed for backwards
compatibility, and it is recommended to use the 1 argument proc format
for new code.

Since you are given all of the records, you can do things like filter on
associations that are specified by multiple keys, or do multiple
queries depending on the content of the records (which would be
necessary for polymorphic associations).  Inside the <tt>:eager_loader</tt>
proc, you should get the related objects and populate the
associations cache for all objects in the array of records.  The hash
of dependent associations is available for you to cascade the eager
loading down multiple levels, but it is up to you to use it.  The
key_hash is a performance enhancement that is used by the default
association loaders and is also available to you.  It is a hash with keys being
foreign/primary key symbols in the current table, and the values
being hashes where the key is foreign/primary key values and values
being arrays of current model objects having the foreign/primary key
value associated with the key.  This is hard to visualize, so I'll
give an example:
    
  album1 = Album.load(:id=>1, :artist_id=>2)
  album2 = Album.load(:id=>3, :artist_id=>2)
  Album.many_to_one :artist
  Album.one_to_many :tracks
  Album.eager(:band, :tracks).all
  # The key_hash provided to the :eager_loader proc would be:
  {:id=>{1=>[album1], 3=>[album2]}, :artist_id=>{2=>[album1, album2]}}

Using these options, you can build associations that Sequel doesn't natively support,
and still be able to use the same eager loading features that you are used to.

== ActiveRecord associations

Sequel supports all of associations that ActiveRecord supports, though some
require different approaches or custom <tt>:eager_loader</tt> options.

=== Association callbacks

Sequel supports the same callbacks that ActiveRecord does on +one_to_many+ and
+many_to_many+ associations: <tt>:before_add</tt>, <tt>:before_remove</tt>, <tt>:after_add</tt>, and
<tt>:after_remove</tt>. One +many_to_one+ associations and +one_to_one+ associations, Sequel
supports the <tt>:before_set</tt> and <tt>:after_set</tt> callbacks.  On all associations,
Sequel supports <tt>:after_load</tt>, which is called after the association has been
loaded.

Each of these options can be a symbol specifying an instance method
that takes one argument (the associated object), or a proc that takes
two arguments (the current object and the associated object), or an
array of symbols and procs.  For <tt>:after_load</tt> with a *_to_many association,
the associated object argument is an array of associated objects.

If any of the before callbacks return +false+, the adding/removing
does not happen and it either raises a <tt>Sequel::BeforeHookFailed</tt> (the default), or
returns false (if +raise_on_save_failure+ is false).

=== Association extensions

All associations come with an <tt><i>association</i>_dataset</tt> method that can be further filtered or
otherwise modified:

  class Author < Sequel::Model
    one_to_many :authorships
  end
  Author.first.authorships_dataset.filter{|o| o.number < 10}.first
 
You can extend a dataset with a module using the <tt>:extend</tt> association option.  You can reference
the model object that created the association dataset via the dataset's
+model_object+ method, and the related association reflection via the dataset's
+association_reflection+ method:

  module FindOrCreate
    def find_or_create(vals)
      first(vals) || model.create(vals.merge(association_reflection[:key]=>model_object.id))
    end
  end
  class Author < Sequel::Model
    one_to_many :authorships, :extend=>FindOrCreate
  end
  Author.first.authorships_dataset.find_or_create(:name=>'Blah', :number=>10)

=== <tt>has_many :through</tt> associations

+many_to_many+ handles the usual case of a <tt>has_many :through</tt> with a +belongs_to+ in
the associated model.  It doesn't break on the case where the join table is a
model table, unlike ActiveRecord's +has_and_belongs_to_many+.

ActiveRecord:

  class Author < ActiveRecord::Base
    has_many :authorships
    has_many :books, :through => :authorships
  end

  class Authorship < ActiveRecord::Base
    belongs_to :author
    belongs_to :book
  end

  @author = Author.find :first
  @author.books

Sequel::Model:

  class Author < Sequel::Model
    one_to_many :authorships
    many_to_many :books, :join_table=>:authorships
  end

  class Authorship < Sequel::Model
    many_to_one :author
    many_to_one :book
  end

  @author = Author.first
  @author.books

If you use an association other than +belongs_to+ in the associated model, such as a +has_many+,
you still use a +many_to_many+ association, but you need to use some options:

ActiveRecord:

  class Firm < ActiveRecord::Base
    has_many :clients
    has_many :invoices, :through => :clients
  end

  class Client < ActiveRecord::Base
    belongs_to :firm
    has_many :invoices
  end

  class Invoice < ActiveRecord::Base
    belongs_to :client
    has_one :firm, :through => :client
  end

  Firm.find(:first).invoices
  
Sequel::Model:

  class Firm < Sequel::Model
    one_to_many :clients
    many_to_many :invoices, :join_table=>:clients, :right_key=>:id, :right_primary_key=>:client_id
  end 

  class Client < Sequel::Model
    many_to_one :firm
    one_to_many :invoices
  end

  class Invoice < Sequel::Model
    many_to_one :client
  end

  Firm.first.invoices

=== Polymorphic Associations

Sequel discourages the use of polymorphic associations, which is the reason they
are not supported by default.  All polymorphic associations can be made non-polymorphic
by using additional tables and/or columns instead of having a column
containing the associated class name as a string.

Polymorphic associations break referential integrity and are significantly more
complex than non-polymorphic associations, so their use is not recommended unless
you are stuck with an existing design that uses them.

If you must use them, look for the sequel_polymorphic plugin, as it makes using
polymorphic associations in Sequel about as easy as it is in ActiveRecord.  However,
here's how they can be done using Sequel's custom associations (the sequel_polymorphic
plugin is just a generic version of this code):

ActiveRecord:

  class Asset < ActiveRecord::Base
    belongs_to :attachable, :polymorphic => true
  end

  class Post < ActiveRecord::Base
    has_many :assets, :as => :attachable
  end

  class Note < ActiveRecord::Base
    has_many :assets, :as => :attachable
  end

  @asset.attachable = @post
  @asset.attachable = @note

Sequel::Model:

  class Asset < Sequel::Model
    many_to_one :attachable, :reciprocal=>:assets, \
      :dataset=>(proc do
        klass = attachable_type.constantize
        klass.filter(klass.primary_key=>attachable_id)
      end), \
      :eager_loader=>(proc do |eo|
        id_map = {}
        eo[:rows].each do |asset|
          asset.associations[:attachable] = nil 
          ((id_map[asset.attachable_type] ||= {})[asset.attachable_id] ||= []) << asset
        end
        id_map.each do |klass_name, id_map|
          klass = klass_name.constantize
          klass.filter(klass.primary_key=>id_map.keys).all do |attach|
            id_map[attach.pk].each do |asset|
              asset.associations[:attachable] = attach
            end
          end
        end
      end)
  
    private

    def _attachable=(attachable)
      self[:attachable_id] = (attachable.pk if attachable)
      self[:attachable_type] = (attachable.class.name if attachable)
    end 
  end 

  class Post < Sequel::Model
    one_to_many :assets, :key=>:attachable_id do |ds|
      ds.filter(:attachable_type=>'Post')
    end 
  
    private

    def _add_asset(asset)
      asset.attachable_id = pk
      asset.attachable_type = 'Post'
      asset.save
    end 
    def _remove_asset(asset)
      asset.attachable_id = nil 
      asset.attachable_type = nil 
      asset.save
    end 
    def _remove_all_assets
      Asset.filter(:attachable_id=>pk, :attachable_type=>'Post')\
        .update(:attachable_id=>nil, :attachable_type=>nil)
    end
  end

  class Note < Sequel::Model
    one_to_many :assets, :key=>:attachable_id do |ds|
      ds.filter(:attachable_type=>'Note')
    end

    private

    def _add_asset(asset)
      asset.attachable_id = pk
      asset.attachable_type = 'Note'
      asset.save
    end
    def _remove_asset(asset)
      asset.attachable_id = nil
      asset.attachable_type = nil
      asset.save
    end
    def _remove_all_assets
      Asset.filter(:attachable_id=>pk, :attachable_type=>'Note')\
        .update(:attachable_id=>nil, :attachable_type=>nil)
    end
  end

  @asset.attachable = @post
  @asset.attachable = @note

== Other advanced associations

=== Joining on multiple keys

Let's say you have two tables that are associated with each other with multiple
keys.  This can be handled using Sequel's built in composite key support for
associations:

  # Both of these models have an album_id, number, and disc_number fields.
  # All FavoriteTracks have an associated track, but not all tracks have an
  # associated favorite track
  
  class Track < Sequel::Model
    many_to_one :favorite_track, :key=>[:disc_number, :number, :album_id], :primary_key=>[:disc_number, :number, :album_id]
  end
  class FavoriteTrack < Sequel::Model
    one_to_one :tracks, :key=>[:disc_number, :number, :album_id], :primary_key=>[:disc_number, :number, :album_id]
  end

=== Tree - All Ancestors and Descendents

Let's say you want to store a tree relationship in your database, it's pretty
simple:

  class Node < Sequel::Model
    many_to_one :parent, :class=>self
    one_to_many :children, :key=>:parent_id, :class=>self
  end
  
You can easily get a node's parent with node.parent, and a node's children with
node.children.  You can even eager load the relationship up to a certain depth:

  # Eager load three generations of generations of children for a given node 
  Node.filter(:id=>1).eager(:children=>{:children=>:children}).all.first
  # Load parents and grandparents for a group of nodes
  Node.filter{|o| o.id < 10}.eager(:parent=>:parent).all

What if you want to get all ancestors up to the root node, or all descendents,
without knowing the depth of the tree?

  class Node < Sequel::Model
    many_to_one :ancestors, :class=>self,
     :eager_loader=>(proc do |eo|
      # Handle cases where the root node has the same parent_id as primary_key
      # and also when it is NULL
      non_root_nodes = eo[:rows].reject do |n| 
        if [nil, n.pk].include?(n.parent_id)
          # Make sure root nodes have their parent association set to nil
          n.associations[:parent] = nil 
          true
        else
          false
        end 
      end 
      unless non_root_nodes.empty?
        id_map = {}
        # Create an map of parent_ids to nodes that have that parent id
        non_root_nodes.each{|n| (id_map[n.parent_id] ||= []) << n}
        # Doesn't cause an infinte loop, because when only the root node
        # is left, this is not called.
        Node.filter(Node.primary_key=>id_map.keys).eager(:ancestors).all do |node|
          # Populate the parent association for each node
          id_map[node.pk].each{|n| n.associations[:parent] = node}
        end 
      end 
    end)
    many_to_one :descendants, :eager_loader=>(proc do |eo|
      id_map = {}
      eo[:rows].each do |n| 
        # Initialize an empty array of child associations for each parent node
        n.associations[:children] = []
        # Populate identity map of nodes
        id_map[n.pk] = n 
      end 
      # Doesn't cause an infinite loop, because the :eager_loader is not called
      # if no records are returned.  Exclude id = parent_id to avoid infinite loop
      # if the root note is one of the returned records and it has parent_id = id
      # instead of parent_id = NULL.
      Node.filter(:parent_id=>id_map.keys).exclude(:id=>:parent_id).eager(:descendants).all do |node|
        # Get the parent from the identity map
        parent = id_map[node.parent_id]
        # Set the child's parent association to the parent 
        node.associations[:parent] = parent
        # Add the child association to the array of children in the parent
        parent.associations[:children] << node
      end 
    end)
  end

Note that unlike ActiveRecord, Sequel supports common table expressions, which allows you to use recursive queries.
The results are not the same as in the above case, as all descendents are stored in a single association,
but all descendants can be both lazy loaded or eager loaded in a single query (assuming your database
supports recursive common table expressions).  Sequel ships with an +rcte_tree+ plugin that makes
this easy:

  class Node < Sequel::Model
    plugin :rcte_tree
  end

=== Joining multiple keys to a single key, through a third table

Let's say you have a database, of songs, lyrics, and artists.  Each song
may or may not have a lyric (most songs are instrumental).  The lyric can be
associated to an artist in each of four ways: composer, arranger, vocalist,
or lyricist.  These may all be the same, or they could all be different, and
none of them are required.  The songs table has a lyric_id field to associate
it to the lyric, and the lyric table has four fields to associate it to the
artist (composer_id, arranger_id, vocalist_id, and lyricist_id).

What you want to do is get all songs for a given artist, ordered by the song's
name, with no duplicates?

  class Artist < Sequel::Model
    one_to_many :songs, :order=>:songs__name, \
      :dataset=>proc{Song.select(:songs.*).join(Lyric, :id=>:lyric_id, id=>[:composer_id, :arranger_id, :vocalist_id, :lyricist_id])}, \
      :eager_loader=>(proc do |eo|
        h = eo[:key_hash][:id]
        ids = h.keys
        eo[:rows].each{|r| r.associations[:songs] = []}
        Song.select(:songs.*, :lyrics__composer_id, :lyrics__arranger_id, :lyrics__vocalist_id, :lyrics__lyricist_id)\
         .join(Lyric, :id=>:lyric_id){{:composer_id=>ids, :arranger_id=>ids, :vocalist_id=>ids, :lyricist_id=>ids}.sql_or}\
         .order(:songs__name).all do |song|
          [:composer_id, :arranger_id, :vocalist_id, :lyricist_id].each do |x|
            recs = h[song.values.delete(x)]
            recs.each{|r| r.associations[:songs] << song} if recs
          end
        end
        eo[:rows].each{|r| r.associations[:songs].uniq!}
      end)
  end

=== Statistics Associations (Sum of Associated Table Column)

In addition to getting associated records, you can use Sequel's association support
to get aggregate information for columns in associated tables (sums, averages, etc.).

Let's say you have a database with projects and tickets.  A project can have many
tickets, and each ticket has a number of hours associated with it.  You can use the
association support to create a Project association that gives the sum of hours for all
associated tickets.

  class Project < Sequel::Model
    one_to_many :tickets
    many_to_one :ticket_hours, :read_only=>true, :key=>:id,
     :dataset=>proc{Ticket.filter(:project_id=>id).select{sum(hours).as(hours)}},
     :eager_loader=>(proc do |eo|
      eo[:rows].each{|p| p.associations[:ticket_hours] = nil}
      Ticket.filter(:project_id=>eo[:key_hash][:id].keys).
       group(:project_id).
       select{[project_id, sum(hours).as(hours)]}.
       all do |t|
        p = eo[:key_hash][:id][t.values.delete(:project_id)].first
        p.associations[:ticket_hours] = t
       end
     end)
    # The association method returns a Ticket object with a single aggregate
    # sum-of-hours value, but you want it to return an Integer/Float of just the
    # sum of hours, so you call super and return just the sum-of-hours value.
    # This works for both lazy loading and eager loading.
    def ticket_hours
      if s = super
        s[:hours]
      end
    end
  end
  class Ticket < Sequel::Model
    many_to_one :project
  end

Note that it is often better to use a sum cache instead of this approach.  You can implement
a sum cache using +after_create+ and +after_delete+ hooks, or using a database trigger
(the preferred method if you only have to support one database and that database supports
triggers).
