Relations on composite keys using sqlalchemy
I have this simple model of Author - Books and can't find a way to make firstName and lastName a composite key and use it in relation. Any ideas?
from sqlalchemy import create_engine, ForeignKey, Column, String, Integer
from sqlalchemy.orm import relationship, sessionmaker
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
engine = create_engine('mssql://user:pass@library')
engine.echo = True
session = sessionmaker(engine)()
class Author(Base):
__tablename__ = 'authors'
firstName = Column(String(20), primary_key=True)
lastName = Column(String(20), primary_key=True)
books = relationship('Book', backref='author')
class Book(Base):
__tablename__ = 'books'
title = Column(String(20), primary_key=True)
author_firstName = Column(String(20), ForeignKey('authors.firstName'))
author_lastName = Column(String(20), ForeignKey('authors.lastName'))
The problem is that you have defined each of the dependent columns as foreign keys separately, when that's not really what you intend, you of course want a composite foreign key. Sqlalchemy is responding to this by saying (in a not very clear way), that it cannot guess which foreign key to use (firstName
or lastName
).
The solution, declaring a composite foreign key, is a tad clunky in declarative, but still fairly obvious:
class Book(Base):
__tablename__ = 'books'
title = Column(String(20), primary_key=True)
author_firstName = Column(String(20))
author_lastName = Column(String(20))
__table_args__ = (ForeignKeyConstraint([author_firstName, author_lastName],
[Author.firstName, Author.lastName]),
{})
The important thing here is that the ForeignKey
definitions are gone from the individual columns, and a ForeignKeyConstraint
is added to a __table_args__
class variable. With this, the relationship
defined on Author.books
works just right.