使用 sqlalchemy 的复合键关系 [英] Relations on composite keys using sqlalchemy

问题描述

我有这个简单的 Author - Books 模型，但找不到使 firstName 和 lastName 成为复合键并将其关联使用的方法.有什么想法吗?

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'))            

推荐答案

问题是你已经将每个依赖列分别定义为外键，如果这不是你真正想要的，你当然想要一个复合外键.Sqlalchemy 对此回应说(以不太清楚的方式)，它无法猜测使用哪个外键(firstName 或 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).

声明复合外键的解决方案在声明中有点笨拙，但仍然相当明显:

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]),
                      {})

这里重要的是 ForeignKey 定义从各个列中消失，并且 ForeignKeyConstraint 添加到 __table_args__ 类变量.有了这个，Author.books 上定义的 relationship 工作得恰到好处.

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.

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