[英]sqlalchemy foreignkey relation with parent class
I have the following database schema: 我有以下数据库架构:
Parent table : 父表 :
Child table - inherits Parent: 子表 -继承父表 :
Summing up I have two tables. 总结一下,我有两个桌子。 Table Child inherits from Parent and also have a foreignkey to it.
Table Child继承自Parent,并且也有一个外键。
UPD: I really need both inheritance and foreignkey . UPD:我真的既需要继承又需要Foreignkey 。 This example is only a short demo which reproduces the problem.
这个例子只是一个简短的演示,它再现了问题。
I used declarative_base to declare the schema: 我用declarative_base声明了模式:
# -*- coding: utf-8 -*-
from sqlalchemy import Column, String, Integer, ForeignKey
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
Base = declarative_base()
class Parent(Base):
__tablename__ = "Parent"
id = Column(Integer, primary_key=True)
type = Column(String(250))
name = Column(String(250))
__mapper_args__ = {
'polymorphic_identity':'Parent',
'polymorphic_on':type
}
class Child(Parent):
__tablename__ = 'Child'
id = Column(Integer, ForeignKey('Parent.id'), primary_key=True)
parent_id = Column(ForeignKey("Parent.id"), nullable=True)
category = Column(String(250))
__mapper_args__ = {
'polymorphic_identity':'Child',
}
engine = create_engine('postgresql+psycopg2://joe:joe@localhost/alch')
session = sessionmaker()
session.configure(bind=engine)
Base.metadata.create_all(engine)
But when I run the code I get the following error: 但是,当我运行代码时,出现以下错误:
sqlalchemy.exc.AmbiguousForeignKeysError: Can't determine join between 'Parent' and 'Child';
sqlalchemy.exc.AmbiguousForeignKeysError:无法确定“父母”和“孩子”之间的联接; tables have more than one foreign key constraint relationship between them.
表之间有多个外键约束关系。 Please specify the 'onclause' of this join explicitly.
请明确指定此联接的“ onclause”。
I have tried to set relationship attribute myself for Parent or for Child separately and for both too. 我尝试分别为父级或子级以及为这两者设置关系属性。 Tried to use primaryjoin and foreign_keys parameters of relationship.
试图使用primaryjoin和foreign_keys参数的关系。 But the error was the same.
但是错误是相同的。
I'm totally confused about this error. 我对此错误完全感到困惑。 I need help.
我需要帮助。
Have you tried removing the Foreign Key for the Child id field? 您是否尝试过删除“儿童ID”字段的外键?
id = Column(Integer, ForeignKey('Parent.id'), primary_key=True)
parent_id = Column(ForeignKey("Parent.id"), nullable=True)
You need something like this: 您需要这样的东西:
id = Column(Integer, auto_increment=True, primary_key=True)
parent_id = Column(ForeignKey("Parent.id"), nullable=True)
I found the solution here . 我在这里找到了解决方案。
SQLAlchemy needs a hint in this situation: a inherit_condition field in Child's __mapper_args__ does the trick. 在这种情况下,SQLAlchemy需要一个提示:Child的__mapper_args__中的native_condition字段可以解决问题。
# -*- coding: utf-8 -*-
from sqlalchemy import Column, String, Integer, ForeignKey
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
Base = declarative_base()
class Parent(Base):
__tablename__ = "Parent"
id = Column(Integer, primary_key=True)
type = Column(String(250))
name = Column(String(250))
__mapper_args__ = {
'polymorphic_identity':'Parent',
'polymorphic_on':type
}
class Child(Parent):
__tablename__ = 'Child'
id = Column(Integer, ForeignKey('Parent.id'), primary_key=True)
parent_id = Column(ForeignKey("Parent.id"), nullable=True)
category = Column(String(250))
parent = relationship(Parent, foreign_keys=[parent_id])
__mapper_args__ = {
'polymorphic_identity':'Child',
'inherit_condition': id == Parent.id,
}
engine = create_engine('postgresql+psycopg2://joe:joe@localhost/alch')
session = sessionmaker()
session.configure(bind=engine)
Base.metadata.create_all(engine)
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