Question

我正在尝试了解如何在SQLAlchemy上使用复合外键进行连接，而我尝试这样做的尝试失败了。

我的玩具模型上有以下模型类（我使用的是Flask-SQLAlchemy，但我不确定这与问题有什么关系）：

# coding=utf-8
from flask import Flask
from flask.ext.sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:////tmp/test.db'
db = SQLAlchemy(app)


class Asset(db.Model):
    __tablename__ = 'asset'
    user = db.Column('usuario', db.Integer, primary_key=True)
    profile = db.Column('perfil', db.Integer, primary_key=True)
    name = db.Column('nome', db.Unicode(255))

    def __str__(self):
        return u"Asset({}, {}, {})".format(self.user, self.profile, self.name).encode('utf-8')


class Zabumba(db.Model):
    __tablename__ = 'zabumba'

    db.ForeignKeyConstraint(
        ['asset.user', 'asset.profile'],
        ['zabumba.user', 'zabumba.profile']
    )

    user = db.Column('usuario', db.Integer, primary_key=True)
    profile = db.Column('perfil', db.Integer, primary_key=True)
    count = db.Column('qtdade', db.Integer)

    def __str__(self):
        return u"Zabumba({}, {}, {})".format(self.user, self.profile, self.count).encode('utf-8')

然后我用一些假数据填充数据库：

db.drop_all()
db.create_all()

db.session.add(Asset(user=1, profile=1, name=u"Pafúncio"))
db.session.add(Asset(user=1, profile=2, name=u"Skavurska"))
db.session.add(Asset(user=2, profile=1, name=u"Ermengarda"))

db.session.add(Zabumba(user=1, profile=1, count=10))
db.session.add(Zabumba(user=1, profile=2, count=11))
db.session.add(Zabumba(user=2, profile=1, count=12))

db.session.commit()

并尝试了以下查询：

> for asset, zabumba in db.session.query(Zabumba).join(Asset).all():
>     print "{:25}\t<---->\t{:25}".format(asset, zabumba)

但SQLAlchemy告诉我，它无法为此连接找到足够的外键：

Traceback (most recent call last):
  File "sqlalchemy_join.py", line 65, in <module>
    for asset, zabumba in db.session.query(Zabumba).join(Asset).all():
  File "/home/calsaverini/.virtualenvs/recsys/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 1724, in join
    from_joinpoint=from_joinpoint)
  File "<string>", line 2, in _join
  File "/home/calsaverini/.virtualenvs/recsys/local/lib/python2.7/site-packages/sqlalchemy/orm/base.py", line 191, in generate
    fn(self, *args[1:], **kw)
  File "/home/calsaverini/.virtualenvs/recsys/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 1858, in _join
    outerjoin, create_aliases, prop)
  File "/home/calsaverini/.virtualenvs/recsys/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 1928, in _join_left_to_right
    self._join_to_left(l_info, left, right, onclause, outerjoin)
  File "/home/calsaverini/.virtualenvs/recsys/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2056, in _join_to_left
    "Tried joining to %s, but got: %s" % (right, ae))
sqlalchemy.exc.InvalidRequestError: Could not find a FROM clause to join from.  Tried joining to <class '__main__.Asset'>, but got: Can't find any foreign key relationships between 'zabumba' and 'asset'.

我尝试了很多其他方法：在两个表上声明ForeignKeyConstraint或将查询反转为db.session.query(Asset).join(Zabumba).all()。

我做错了什么？

感谢。

P.S。：在我的实际应用程序代码中，问题实际上有点复杂，因为这些表在不同的模式上，我将使用绑定的东西：

app.config['SQLALCHEMY_BINDS'] = {
    'assets':        'mysql+mysqldb://fooserver/assets',
    'zabumbas':      'mysql+mysqldb://fooserver/zabumbas',
}

然后我将声明表格上的不同绑定。我应该如何声明ForeignKeyConstraint呢？

Answer 1

你的代码中只有很少的拼写错误，纠正这将使整个代码工作。

正确定义ForeignKeyConstraint：

不仅要定义它，您必须将其添加到__table_args__
列和 refcolumns 参数的定义相反（参见documentation）
列的名称必须是数据库中的名称，而不是ORM属性的名称

如下面的代码所示：

class Zabumba(db.Model):
    __tablename__ = 'zabumba'

    __table_args__ = (
        db.ForeignKeyConstraint(
            ['usuario', 'perfil'],
            ['asset.usuario', 'asset.perfil'],
        ),
    )

通过在查询子句中包含两个类来正确构造查询：

    for asset, zabumba in db.session.query(Asset, Zabumba).join(Zabumba).all():
        print "{:25}\t<---->\t{:25}".format(asset, zabumba)

SQLAlchemy连接复合外键（使用flask-sqlalchemy）

1 个答案: