SQLAlchemy - 如何使eager loading count属性

Question

我想为包含计数的模型创建一个属性.

因为我总是需要的财产,我想使查询与JOIN类似sqlalchemy.orm.relationship与lazy='joined'

例如,我定义了如下的模型

import sqlalchemy as s, func
from sqlalchemy.orm import relatioship

# ...

class Foo(Base):
    __tablename__ = 'foo'
    id = s.Column(s.Integer, primary_key=True)
    bar_id = s.Column(s.Integer, s.ForeignKey('bar.id'))
    bar = relationship('Bar')


class Bar(Base):
    __tablename__ = 'bar'
    id = s.Column(s.Integer, primary_key=True)

    @property
    def foo_count(self):
        return Foo.query.filter_by(bar=self).count()

当我访问属性时foo_count,它会向DBMS发送查询.

因为我总是访问这个属性,我想急切地加载它这样的计数属性

# Not session.query(Bar, func.count(Foo.id)).join(Foo) ...
bar = Bar.query.first()

SQL将是这样的

SELECT id, COUNT(Foo.id)
FROM bar 
INNER JOIN foo
    ON bar.id = foo.id

然后bar.foo_count就不会发生SQL查询.

我该如何制作一个属性foo_count？

Answer 1

我用它来解决它 sqlalchemy.orm.column_property

我替换了foo_count以下

import sqlalchemy as s, func, select
from sqlalchemy.orm import relationship, column_property

# ...

class Foo(Base):
    __tablename__ = 'foo'
    id = s.Column(s.Integer, primary_key=True)
    bar_id = s.Column(s.Integer, s.ForeignKey('bar.id'))
    bar = relationship('Bar')


class Bar(Base):
    __tablename__ = 'bar'
    id = s.Column(s.Integer, primary_key=True)

    foo_count = column_property(
        select([func.count(Foo.id)])
        .where(Foo.bar_id == id)
    )

Answer 2

请查看Hybrid Attribute扩展.

您的对象模型将类似于以下内容:

class Foo(Base):
    __tablename__ = 'foo'
    id = Column(Integer, primary_key=True)
    bar_id = Column(Integer, ForeignKey('bar.id'))
    bar = relationship('Bar')

class Bar(Base):
    __tablename__ = 'bar'
    id = Column(Integer, primary_key=True)

    @hybrid_property
    def foo_count(self):
        return object_session(self).query(Foo).filter(Foo.bar==self).count()

    @foo_count.expression
    def foo_count(cls):
        return select([func.count(Foo.id)]).where(Foo.bar_id == cls.id).label('foo_count')

foo_count不会急切加载,但你可以在下面的查询中使用它(在in SELECT和in WHERE子句中:

qry = session.query(Bar, Bar.foo_count).filter(Bar.foo_count > 0)
for (bar, bar_foo_count) in qry:
    print bar, bar_foo_count

正如您所看到的,查询将仅返回(Bar, foo_count)一个查询中的元组,现在您可以使用该查询执行所需的操作.