add_constraints

# File activerecord/lib/active_record/associations/association_scope.rb, line 36
      def add_constraints(scope)
        tables = construct_tables

        chain.each_with_index do |reflection, i|
          table, foreign_table = tables.shift, tables.first

          if reflection.source_macro == :has_and_belongs_to_many
            join_table = tables.shift

            scope = scope.joins(join(
              join_table,
              table[reflection.association_primary_key].
                eq(join_table[reflection.association_foreign_key])
            ))

            table, foreign_table = join_table, tables.first
          end

          if reflection.source_macro == :belongs_to
            if reflection.options[:polymorphic]
              key = reflection.association_primary_key(klass)
            else
              key = reflection.association_primary_key
            end

            foreign_key = reflection.foreign_key
          else
            key         = reflection.foreign_key
            foreign_key = reflection.active_record_primary_key
          end

          conditions = self.conditions[i]

          if reflection == chain.last
            scope = scope.where(table[key].eq(owner[foreign_key]))

            if reflection.type
              scope = scope.where(table[reflection.type].eq(owner.class.base_class.name))
            end

            conditions.each do |condition|
              if options[:through] && condition.is_a?(Hash)
                condition = disambiguate_condition(table, condition)
              end

              scope = scope.where(interpolate(condition))
            end
          else
            constraint = table[key].eq(foreign_table[foreign_key])

            if reflection.type
              type = chain[i + 1].klass.base_class.name
              constraint = constraint.and(table[reflection.type].eq(type))
            end

            scope = scope.joins(join(foreign_table, constraint))

            unless conditions.empty?
              scope = scope.where(sanitize(conditions, table))
            end
          end
        end

        scope
      end