1 files changed, 80 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/associations/through_association.rb b/activerecord/lib/active_record/associations/through_association.rb
new file mode 100644
index 0000000000..e6ab628719
--- /dev/null
+++ b/activerecord/lib/active_record/associations/through_association.rb
@@ -0,0 +1,80 @@
+module ActiveRecord
+  # = Active Record Through Association
+  module Associations
+    module ThroughAssociation #:nodoc:
+
+      delegate :source_reflection, :through_reflection, :chain, :to => :reflection
+
+      protected
+
+        # We merge in these scopes for two reasons:
+        #
+        #   1. To get the default_scope conditions for any of the other reflections in the chain
+        #   2. To get the type conditions for any STI models in the chain
+        def target_scope
+          scope = super
+          chain[1..-1].each do |reflection|
+            scope = scope.merge(reflection.klass.scoped)
+          end
+          scope
+        end
+
+      private
+
+        # Construct attributes for :through pointing to owner and associate. This is used by the
+        # methods which create and delete records on the association.
+        #
+        # We only support indirectly modifying through associations which has a belongs_to source.
+        # This is the "has_many :tags, :through => :taggings" situation, where the join model
+        # typically has a belongs_to on both side. In other words, associations which could also
+        # be represented as has_and_belongs_to_many associations.
+        #
+        # We do not support creating/deleting records on the association where the source has
+        # some other type, because this opens up a whole can of worms, and in basically any
+        # situation it is more natural for the user to just create or modify their join records
+        # directly as required.
+        def construct_join_attributes(*records)
+          if source_reflection.macro != :belongs_to
+            raise HasManyThroughCantAssociateThroughHasOneOrManyReflection.new(owner, reflection)
+          end
+
+          join_attributes = {
+            source_reflection.foreign_key =>
+              records.map { |record|
+                record.send(source_reflection.association_primary_key)
+              }
+          }
+
+          if options[:source_type]
+            join_attributes[source_reflection.foreign_type] =
+              records.map { |record| record.class.base_class.name }
+          end
+
+          if records.count == 1
+            Hash[join_attributes.map { |k, v| [k, v.first] }]
+          else
+            join_attributes
+          end
+        end
+
+        # Note: this does not capture all cases, for example it would be crazy to try to
+        # properly support stale-checking for nested associations.
+        def stale_state
+          if through_reflection.macro == :belongs_to
+            owner[through_reflection.foreign_key].to_s
+          end
+        end
+
+        def foreign_key_present?
+          through_reflection.macro == :belongs_to &&
+          !owner[through_reflection.foreign_key].nil?
+        end
+
+        def ensure_not_nested
+          if reflection.nested?
+            raise HasManyThroughNestedAssociationsAreReadonly.new(owner, reflection)
+          end
+        end
+    end
+  end
+end