SQL Alchemy Closure表关系定义

Question

我最近开始使用SQL Alchemy进行涉及攀爬区域和路线的项目.区域是分层的,因为单个区域可能包含多个区域,而这些区域又可能包含其他区域.路线直接与单个区域相关联,但也与该区域的父级相关联.

为了实现这一点,我选择使用一个封闭表ala Bill Karwin.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,也会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

要查询给定路径的所有区域(在树上),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单个路线的区域.但是,当我尝试在DbArea中使用backref'routes'时,我收到以下错误:

sqlalchemy.exc.StatementError:没有在mapper Mapper | DbArea | area ...上配置列route.area_id(原因:UnmappedColumnError:在mapper Mapper | DbArea | area上没有配置列route.area_id ...)'SELECT路由.route_id AS route_route_id,route.area_id AS route_area_id,route.name AS route_name,route.created AS route_created \nFROM route,area_relationship \nWHERE%s = area_relationship.descendent AND route.area_id = area_relationship.ancestor'[immutabledict({}) ]

我猜我可能需要向DbArea添加一些内容以建立关系,但在尝试了一些不同的选项后无法确定解决方案.

Answer 1

在发布到SQL Alchemy Google Group并获得Michael Bayer的一些非常棒的帮助之后,我得出了以下DbRoute类中区域关系的定义

areas = relationship('DbArea',
    backref = backref('routes', order_by = 'DbRoute.name'),
    secondary = area_relationship_table,
    primaryjoin = area_id == area_relationship_table.c.descendent,
    secondaryjoin = DbArea.area_id == area_relationship_table.c.ancestor,
    innerjoin = True, order_by = DbArea.name,
    foreign_keys =
       [area_relationship_table.c.ancestor,
            area_relationship_table.c.descendent]) 

关键在于正确定义连接.现在,我可以轻松地从路径实例中查找祖先树中的区域,或从区域中查找区域,并查找后代树中的所有路径.