[英]How can I query a sqlalchemy relationship with an int instead of an instance?
我有兩個模型,Foo和Bar。 Bar與Foo具有一對多關系,具有“ foo_id”列和“ foo”關系。 我想查詢具有foo_id = 1的Bar行。
據我所知,有兩種工作方法可以做到這一點:
query.filter(Bar.foo_id == 1)
query.filter(Bar.foo == instance)
我想要這兩種方法的替代方法,更像是query.filter(Bar.foo == 1)
-也就是說,使用關系列,並使用普通整數而不是實例。 當前這會失敗,並出現AttributeError: 'int' object has no attribute '_sa_instance_state'
我想避免上述兩種方法的原因:
第一種方法在實際應用程序中不可行,因為我不知道該foo_id
列的名稱-所有表元數據都是通過反射生成的,並且列名是我不希望依賴的實現細節(它們目前非常不一致,並且正在使用Alembic遷移進行重命名,因此不希望依賴這些名稱)
第二種方法添加了一個SQL查詢的額外往返行程,以獲取Foo的實例,然后我才能執行實際的查詢,該查詢不使用除我已經擁有的整數ID之外的任何東西(並且身份映射不使用用我使用它的方式緩存它)。 我承認這部分很危險地接近過早的優化,但是仍然感到浪費,應該有更好的方法。
解決這個問題的另一種方法是如何從RelationshipProperty開始到達外鍵列,可能使用自省
我也可以用某種方式來獲取某種延遲加載的Foo實例,該實例足以通過ID查詢,但實際上並不發送SQL查詢
這是一些自包含的測試代碼來說明問題:
from sqlalchemy import create_engine, Column, Integer, ForeignKey
from sqlalchemy.orm import relationship, Session
from sqlalchemy.ext.declarative import declarative_base
def method_1(session, some_foo_id):
"""Works, but i don't actually know foo_id"""
session.query(Bar).filter(Bar.foo_id == some_foo_id).first()
def method_2(session, some_foo_id):
"""Works, but adds a pointless roundtrip"""
some_foo = session.query(Foo).get(some_foo_id)
session.query(Bar).filter(Bar.foo == some_foo).first()
def method_3(session, some_foo_id):
"""Throws an exception, passing int instead of instance"""
session.query(Bar).filter(Bar.foo == some_foo_id).first()
# database setup follows
Base = declarative_base()
class Foo(Base):
__tablename__ = 'foo'
id = Column(Integer, primary_key=True)
value = Column(Integer)
class Bar(Base):
__tablename__ = 'bar'
id = Column(Integer, primary_key=True)
foo_id = Column(Integer, ForeignKey('foo.id'))
foo = relationship(Foo)
if __name__ == '__main__':
engine = create_engine('sqlite://')
Base.metadata.create_all(engine)
session = Session(bind=engine)
foo1, foo2 = Foo(value=1), Foo(value=2)
bar1, bar2 = Bar(foo=foo1), Bar(foo=foo2)
session.add_all([foo1, foo2, bar1, bar2])
session.commit()
engine.echo = True
for fun in [method_1, method_2, method_3]:
print("\n---> %s (%s)\n" % (fun.__name__, fun.__doc__))
fun(session, 1)
session.rollback()
輸出:
---> method_1 (Works, but i don't actually know foo_id
BEGIN (implicit)
SELECT bar.id AS bar_id, bar.foo_id AS bar_foo_id
FROM bar
WHERE bar.foo_id = ?
LIMIT ? OFFSET ?
(1, 1, 0)
ROLLBACK
---> method_2 (Works, but adds a pointless roundtrip)
BEGIN (implicit)
SELECT foo.id AS foo_id, foo.value AS foo_value
FROM foo
WHERE foo.id = ?
(1,)
SELECT bar.id AS bar_id, bar.foo_id AS bar_foo_id
FROM bar
WHERE ? = bar.foo_id
LIMIT ? OFFSET ?
(1, 1, 0)
ROLLBACK
---> method_3 (Throws an exception, passing int instead of instance)
Traceback (most recent call last):
File "asd.py", line 51, in <module>
fun(session, 1)
File "asd.py", line 19, in method_3
session.query(Bar).filter(Bar.foo == some_foo_id).first()
File "/usr/lib/python2.7/site-packages/sqlalchemy/sql/operators.py", line 304, in __eq__
return self.operate(eq, other)
File "/usr/lib/python2.7/site-packages/sqlalchemy/orm/attributes.py", line 175, in operate
return op(self.comparator, *other, **kwargs)
File "/usr/lib/python2.7/site-packages/sqlalchemy/orm/relationships.py", line 1042, in __eq__
other, adapt_source=self.adapter))
File "/usr/lib/python2.7/site-packages/sqlalchemy/orm/relationships.py", line 1369, in _optimized_compare
state = attributes.instance_state(state)
AttributeError: 'int' object has no attribute '_sa_instance_state'
找到了一種方法, Bar.foo.prop.local_columns
是RelationshipProperty的屬性,該屬性返回一個通常包含一個項目的集合(對於像這樣的簡單關系)。 集合是無序的,您不能只獲得第一項,所以list(...)[0]
。 完整代碼:
def method_4(session, some_foo_id):
foo_col = list(Bar.foo.prop.local_columns)[0]
session.query(Bar).filter(foo_col == some_foo_id).first()
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