有沒有官方的方式來做到這一點,但下面的代碼爲我生成了以下解決方案。我使用鏈接的docs中的節點示例。
class Node(Base):
__tablename__ = 'node'
id = Column(Integer, primary_key=True)
parent_id = Column(Integer, ForeignKey('node.id'))
data = Column(String(50))
children = relationship("Node",
lazy="joined",
join_depth=2)
在創建時,所述children
屬性被賦予2。這初始值被記錄在一個Node.children.property.join_depth
join_depth
。但是,更改此值不會執行任何操作。初始時,該關係爲加入創建一個「策略」,並複製join_depth
值。 要更改關係策略的連接深度,請設置Node.children.property.strategy.join_depth
。
>>> engine.echo = True # print generated queries
>>> session.query(Node).all() # with default join_depth
SELECT node.id AS node_id, node.parent_id AS node_parent_id, node.data AS node_data, node_1.id AS node_1_id, node_1.parent_id AS node_1_parent_id, node_1.data AS node_1_data, node_2.id AS node_2_id, node_2.parent_id AS node_2_parent_id, node_2.data AS node_2_data FROM node LEFT OUTER JOIN node AS node_2 ON node.id = node_2.parent_id LEFT OUTER JOIN node AS node_1 ON node_2.id = node_1.parent_id
>>> Node.children.property.strategy.join_depth = 4 # new join depth
>>> session.query(Node).all() # with new join depth
SELECT node.id AS node_id, node.parent_id AS node_parent_id, node.data AS node_data, node_1.id AS node_1_id, node_1.parent_id AS node_1_parent_id, node_1.data AS node_1_data, node_2.id AS node_2_id, node_2.parent_id AS node_2_parent_id, node_2.data AS node_2_data, node_3.id AS node_3_id, node_3.parent_id AS node_3_parent_id, node_3.data AS node_3_data, node_4.id AS node_4_id, node_4.parent_id AS node_4_parent_id, node_4.data AS node_4_data FROM node LEFT OUTER JOIN node AS node_4 ON node.id = node_4.parent_id LEFT OUTER JOIN node AS node_3 ON node_4.id = node_3.parent_id LEFT OUTER JOIN node AS node_2 ON node_3.id = node_2.parent_id LEFT OUTER JOIN node AS node_1 ON node_2.id = node_1.parent_id
設定Node.children.property.strategy.join_depth
後,在生成的查詢連接的數目變化爲好。