association_join

# File activerecord/lib/active_record/associations.rb, line 2137
            def association_join
              return @join if @join

              aliased_table = Arel::Table.new(table_name, :as      => @aliased_table_name,
                                                          :engine  => arel_engine,
                                                          :columns => klass.columns)

              parent_table = Arel::Table.new(parent.table_name, :as      => parent.aliased_table_name,
                                                                :engine  => arel_engine,
                                                                :columns => parent.active_record.columns)

              @join = case reflection.macro
              when :has_and_belongs_to_many
                join_table = Arel::Table.new(options[:join_table], :as => aliased_join_table_name, :engine => arel_engine)
                fk = options[:foreign_key] || reflection.active_record.to_s.foreign_key
                klass_fk = options[:association_foreign_key] || klass.to_s.foreign_key

                [
                  join_table[fk].eq(parent_table[reflection.active_record.primary_key]),
                  aliased_table[klass.primary_key].eq(join_table[klass_fk])
                ]
              when :has_many, :has_one
                if reflection.options[:through]
                  join_table = Arel::Table.new(through_reflection.klass.table_name, :as => aliased_join_table_name, :engine => arel_engine)
                  jt_as_extra = jt_source_extra = jt_sti_extra = nil
                  first_key = second_key = as_extra = nil

                  if through_reflection.macro == :belongs_to
                    jt_primary_key = through_reflection.primary_key_name
                    jt_foreign_key = through_reflection.association_primary_key
                  else
                    jt_primary_key = through_reflection.active_record_primary_key
                    jt_foreign_key = through_reflection.primary_key_name

                    if through_reflection.options[:as] # has_many :through against a polymorphic join
                      jt_as_extra = join_table[through_reflection.options[:as].to_s + '_type'].eq(parent.active_record.base_class.name)
                    end
                  end

                  case source_reflection.macro
                  when :has_many, :has_one
                    if source_reflection.options[:as]
                      first_key   = "#{source_reflection.options[:as]}_id"
                      second_key  = options[:foreign_key] || primary_key
                      as_extra    = aliased_table["#{source_reflection.options[:as]}_type"].eq(source_reflection.active_record.base_class.name)
                    else
                      first_key   = through_reflection.klass.base_class.to_s.foreign_key
                      second_key  = options[:foreign_key] || primary_key
                    end

                    unless through_reflection.klass.descends_from_active_record?
                      jt_sti_extra = join_table[through_reflection.active_record.inheritance_column].eq(through_reflection.klass.sti_name)
                    end
                  when :belongs_to
                    first_key = primary_key
                    if reflection.options[:source_type]
                      second_key = source_reflection.association_foreign_key
                      jt_source_extra = join_table[reflection.source_reflection.options[:foreign_type]].eq(reflection.options[:source_type])
                    else
                      second_key = source_reflection.primary_key_name
                    end
                  end

                  [
                    [parent_table[jt_primary_key].eq(join_table[jt_foreign_key]), jt_as_extra, jt_source_extra, jt_sti_extra].reject{|x| x.blank? },
                    [aliased_table[first_key].eq(join_table[second_key]), as_extra].reject{ |x| x.blank? }
                  ]
                elsif reflection.options[:as]
                  id_rel = aliased_table["#{reflection.options[:as]}_id"].eq(parent_table[parent.primary_key])
                  type_rel = aliased_table["#{reflection.options[:as]}_type"].eq(parent.active_record.base_class.name)
                  [id_rel, type_rel]
                else
                  foreign_key = options[:foreign_key] || reflection.active_record.name.foreign_key
                  [aliased_table[foreign_key].eq(parent_table[reflection.options[:primary_key] || parent.primary_key])]
                end
              when :belongs_to
                [aliased_table[options[:primary_key] || reflection.klass.primary_key].eq(parent_table[options[:foreign_key] || reflection.primary_key_name])]
              end

              unless klass.descends_from_active_record?
                sti_column = aliased_table[klass.inheritance_column]
                sti_condition = sti_column.eq(klass.sti_name)
                klass.descendants.each {|subclass| sti_condition = sti_condition.or(sti_column.eq(subclass.sti_name)) }

                @join << sti_condition
              end

              [through_reflection, reflection].each do |ref|
                if ref && ref.options[:conditions]
                  @join << process_conditions(ref.options[:conditions], aliased_table_name)
                end
              end

              @join
            end