sqlalchemy多关联参考模型的过滤

Question

例如，我有一个Parcel模型，它有sender和{}，两者都是Subjects。 我想从特定的寄件人那里得到包裹。我不想使用Parcel.sender.has()，因为性能的原因，我的实际表太大了。在

来自docs：

Because has() uses a correlated subquery, its performance is not nearly as good when compared against large target tables as that of using a join.

下面是一个完整的粘贴和运行示例：

from sqlalchemy import create_engine, Column, Integer, Text, ForeignKey
from sqlalchemy.orm import sessionmaker, relationship
from sqlalchemy.ext.declarative.api import declarative_base
from sqlalchemy.orm.util import aliased

engine = create_engine('sqlite://')
Session = sessionmaker(bind=engine)
s = Session()

Base = declarative_base()


class Subject(Base):
    __tablename__ = 'subject'

    id = Column(Integer, primary_key=True)
    name = Column(Text)


class Parcel(Base):
    __tablename__ = 'parcel'

    id = Column(Integer, primary_key=True)
    sender_id = Column(Integer, ForeignKey('subject.id'))
    receiver_id = Column(Integer, ForeignKey('subject.id'))

    sender = relationship('Subject', foreign_keys=[sender_id], uselist=False, lazy='joined')
    receiver = relationship('Subject', foreign_keys=[receiver_id], uselist=False, lazy='joined')

    def __repr__(self):
        return '<Parcel #{id} {s} -> {r}>'.format(id=self.id, s=self.sender.name, r=self.receiver.name)


# filling database
Base.metadata.create_all(engine)
p = Parcel()
p.sender, p.receiver = Subject(name='Bob'), Subject(name='Alice')
s.add(p)
s.flush()


#
# Method #1 - using `has` method - working but slow
print(s.query(Parcel).filter(Parcel.sender.has(name='Bob')).all())

因此，我尝试按别名关系进行连接和筛选，但出现了一个错误：

^{pr2}$

我发现如果我用连接条件而不是关系来指定模型，它就可以工作了。但最终的SQL查询并不是我所期望的：

print(
    s.query(Parcel)\
    .join(Subject, Parcel.sender_id == Subject.id)\
    .filter(Subject.name == 'Bob')
)

生成下一个SQL查询：

SELECT parcel.id AS parcel_id,
       parcel.sender_id AS parcel_sender_id,
       parcel.receiver_id AS parcel_receiver_id,
       subject_1.id AS subject_1_id,
       subject_1.name AS subject_1_name,
       subject_2.id AS subject_2_id,
       subject_2.name AS subject_2_name
FROM parcel
JOIN subject ON parcel.sender_id = subject.id
LEFT OUTER JOIN subject AS subject_1 ON subject_1.id = parcel.sender_id
LEFT OUTER JOIN subject AS subject_2 ON subject_2.id = parcel.receiver_id
WHERE subject.name = ?

在这里您可以看到subject表被联接了三次，而不是两次。这是因为sender和receiver关系都配置为加载联接。第三个连接是主题，我通过它过滤。在

我希望最终查询如下所示：

SELECT parcel.id AS parcel_id,
       parcel.sender_id AS parcel_sender_id,
       parcel.receiver_id AS parcel_receiver_id,
       subject_1.id AS subject_1_id,
       subject_1.name AS subject_1_name,
       subject_2.id AS subject_2_id,
       subject_2.name AS subject_2_name
FROM parcel
LEFT OUTER JOIN subject AS subject_1 ON subject_1.id = parcel.sender_id
LEFT OUTER JOIN subject AS subject_2 ON subject_2.id = parcel.receiver_id
WHERE subject_1.name = ?

我认为，通过多个引用关系进行过滤不应该如此模糊，而且有更好更清晰的方法来实现。请帮我找一下。在

Answer 1

您将其配置为sender和{}将始终通过join加载。
您可以更改它并手动执行joinedload，当您实际需要使用一个join同时加载它们时。在

如果您希望保持定义不变，您可以“帮助”SQLAlchemy并指出查询已经具有用于此比较的所有数据，并且不需要额外的联接。为此，contains_eager选项被使用。在

修改后的查询：

q = (s.query(Parcel)
     .join(Parcel.sender)
     .options(contains_eager(Parcel.sender))
     .filter(Subject.name == 'Bob'))

它产生的SQL：

^{pr2}$