实体-(多对一)->相关表-（多对一）-->属性映射不起作用

Question

我在一个相当简单的情况下遇到映射问题。有人可以帮忙吗？ 这是 DTO：

class AttributeDTO(object):
    id = None
    name = None

class RelationDTO(object):
    id = None
    name = None
    attribute = None # one attribute per relation

class EntityDTO(object):
    id = None
    name = None

    relation = None
    #it works fine, but then I must get myEnt.relation.attribute, 
    #when i want to get the attribute from entity

    attribute = None # I want it to be placed HERE!

这是映射器：

class TableMapper(object):

    def __init__(self, metadata, mapped_type):
        self._table = None
        self._mapped_type = mapped_type

    def get_table(self):
        return self._table

    def set_table(self, table):
        self._table = table

    def map_table(self):
        mapper(self._mapped_type, self._table)
        return self._table

class AttributeTableMapper(TableMapper):
    ...

class RelationTableMapper(TableMapper):
    ...
    def map_table(self, attribute_table):
        r_attribute = relationship(AttributeDTO, 
            uselist = False, remote_side = [attribute_table.c.id])
    mapper(RelationDTO, 
           self._table, 
           properties = {'attribute': r_attribute})
    return self._table

class EntityTableMapper(TableMapper):

    def __init__(self, metadata):
        TableMapper.__init__(self, metadata, EntityDTO)
        self.set_table(Table('entities', metadata,
                         Column('id', Integer, ...)))

    def map_table(self, relation_table, attribute_table):
        r_attribute = relationship(AtributeDTO, uselist = False, 
#I TRIED THE FOLLOWING, BUT GOT ERRORS
#!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
            primaryjoin = self._table.c.id_relation == relation.c.id,
            secondary = relation_table,
            secondaryjoin = relation_table.c.id_attribute == attribute_table.c.id)
#!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
        mapper(EntityDTO, self._table, properties={'attribute': r_attribute})
        return self._table

因此，我映射表：

attribute_t_mapper = AttributeTableMapper(self._metadata)
relation_table = RelationTableMapper(self._metadata).\
    map_table(attribute_t_mapper.get_table())
attribute_table = attribute_t_mapper.map_table()
entity_table = EntityTableMapper(self._metadata).\
    map_table(relation_table, attribute_table)

并尝试从实体获取我的属性：

entity = session.query(EntityDTO).first() #OK!
a = entity.relation.attribute #OK!
a = entity.attribute #ERROR!

sqlalchemy.orm.exc.UnmappedColumnError: 在映射器 Mapper|EntityDTO|entities 上没有配置列关系.id ...

我做错了什么？