I have the following database schema:
*Table "Parent"*:
1. id - primary key identifier.
2. type - polymorphic_identity.
3. name - string data column.
*Table "Child"* - inherits Parent:
1. id - primary key identifier.
2. parent_id - foreignkey to Parent.
3. category - string data column.
Summing up I have two tables. Table Child inherits from Parent and also
have a foreignkey to it.
I really need both *inheritance* and *foreignkey*. This example is only a
short demo which reproduces the problem.
My real database has 1000+ tables with complex inheritance.
I used declarative_base to declare the schema:
# -*- 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:
sqlalchemy.exc.AmbiguousForeignKeysError: Can't determine join between
> 'Parent' and 'Child'; tables have more than one foreign key constraint
> relationship between them. Please specify the 'onclause' of this join
> explicitly.
I have tried to set *relationship* attribute for Parent or for Child
separately and for both too. Tried to use *primaryjoin* and *foreign_keys*
parameters of relationship. But the error was the same.
I'm totally confused about this error.
Please help. Thanks.
--
SQLAlchemy -
The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
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