through_scope_attributes

# File activerecord/lib/active_record/associations/has_many_through_association.rb, line 71
        def through_scope_attributes
          scope = through_scope || self.scope
          attributes = scope.where_values_hash(through_association.reflection.klass.table_name)
          except_keys = [
            *Array(through_association.reflection.foreign_key),
            through_association.reflection.klass.inheritance_column
          ]
          attributes.except!(*except_keys)
        end