我最近开始使用 SQL Alchemy 开展一个涉及攀岩区域和路线的项目。区域是分层的,因为单个区域可以包含多个区域,而多个区域又可以包含其他区域。路线直接与单个区域关联,但也与该区域的父区域关联,等等。
为了实现这一点,我选择使用Bill Karwin 的闭合表 http://karwin.blogspot.com/2010/03/rendering-trees-with-closure-tables.html。在闭包表实现中,创建第二个表来存储祖先/后代信息。添加节点时会创建自引用行,并为树中的每个祖先创建一行。
表结构如下(简化):
-- area --
area_id
name
-- area_relationship --
ancestor
descendent
-- route --
route_id
area_id
name
样本数据:
-- area --
1, New River Gorge
2, Kaymoor
3, South Nuttall
4, Meadow River Gorge
-- area_relationship (ancestor, descendent) --
1, 1 (self-referencing)
2, 2 (self-referencing)
1, 2 (Kaymoor is w/i New River Gorge)
3, 3 (self-referencing)
1, 3 (South Nutall is w/i New River Gorge)
4, 4 (self-referencing)
-- route (route_id, area_id, name)
1, 2, Leave it to Jesus
2, 2, Green Piece
3, 4, Fancy Pants
要查询给定路线(树上)的所有区域,我可以执行:
SELECT area.area_id, area.name
FROM route
INNER JOIN area_relationship ON route.area_id = area_relationship.descendent
INNER JOIN area ON area.area_id = area_relationship.ancestor
WHERE route.route_id = 1
同样,我可以使用以下命令查询特定区域(包括后代区域)中的所有路线:
SELECT route.route_id, route.name
FROM area
INNER JOIN area_relationship ON area.area_id = area_relationship.ancestor
INNER JOIN route ON route.area_id = area_relationship.descendent
WHERE area.area_id = 1
在 SQL Alchemy 中,我创建了一个关系和两个表来处理这些关系:
area_relationship_table = Table('area_relationship', Base.metadata,
Column('ancestor', Integer, ForeignKey('area.area_id')),
Column('descendent', Integer, ForeignKey('area.area_id'))
)
DbArea 类 -
class DbArea(Base):
__tablename__ = 'area'
area_id = Column(Integer, primary_key = True)
name = Column(VARCHAR(50))
created = Column(DATETIME)
area_relationship_table.c.ancestor])
descendents = relationship('DbArea', backref = 'ancestors',
secondary = area_relationship_table,
primaryjoin = area_id == area_relationship_table.c.ancestor,
secondaryjoin = area_id == area_relationship_table.c.descendent)
DbRoute 类 -
class DbRoute(Base):
__tablename__ = 'route'
route_id = Column(Integer, primary_key = True)
area_id = Column(Integer, ForeignKey('area.area_id'))
name = Column(VARCHAR(50))
created = Column(DATETIME)
area = relationship("DbArea")
areas = relationship('DbArea', backref = 'routes',
secondary = area_relationship_table,
primaryjoin = area_id == area_relationship_table.c.ancestor,
secondaryjoin = area_id == area_relationship_table.c.descendent,
foreign_keys=[area_relationship_table.c.ancestor,
area_relationship_table.c.descendent])
目前,我可以使用 DbRoute 中的区域关系来确定各个路线的区域。但是,当我尝试在 DbArea 中使用 backref 'routes' 时,出现以下错误:
sqlalchemy.exc.StatementError: 在映射器Mapper|DbArea|area... 上没有配置列route.area_id... (原始原因:UnmappedColumnError: 在映射器Mapper|DbArea|area... 上没有配置列route.area_id...) 'SELECT 路由.route_id AS 路由_路由_id、路由.area_id AS 路由_区域_id、路由.name AS 路由_名称、route.created AS 路由_创建 \nFROM 路由、area_relationship \nWHERE %s = area_relationship.descendent AND route.area_id = area_relationship.ancestor' [immutabledict({}) ]
我猜测我可能需要向 DbArea 添加一些内容来建立关系,但在尝试了一些不同的选项后无法确定解决方案。