我正在尝试建立一个跨越四个表的关系。我根据this question中的代码简化了我的代码以匹配我的数据库。
from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class A(Base):
__tablename__ = 'a'
id = Column(Integer, primary_key=True)
b_id = Column(Integer, ForeignKey('b.id'))
# FIXME: This fails with:
# "Relationship A.ds could not determine any unambiguous local/remote column pairs based on
# join condition and remote_side arguments. Consider using the remote() annotation to
# accurately mark those elements of the join condition that are on the remote side of the relationship."
#
# ds = relationship("D", primaryjoin="and_(A.b_id == B.id, B.id == C.b_id, D.id == C.d_id)", viewonly=True)
def dq(self):
return sess.query(D).filter(and_(D.id == C.d_id,
C.b_id == B.id,
B.id == A.id,
A.id == self.id))
class B(Base):
__tablename__ = 'b'
id = Column(Integer, primary_key=True)
class C(Base):
__tablename__ = 'c'
b_id = Column(Integer, ForeignKey('b.id'), primary_key=True)
d_id = Column(Integer, ForeignKey('d.id'), primary_key=True)
class D(Base):
__tablename__ = 'd'
id = Column(Integer, primary_key=True)
e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)
sess = Session(e)
sess.add(D(id=1))
sess.add(D(id=2))
sess.add(B(id=1))
sess.add(C(b_id=1, d_id=1))
sess.add(C(b_id=1, d_id=2))
sess.add(A(id=1, b_id=1))
sess.flush()
a1 = sess.query(A).first()
print a1.dq().all()
#print a1.ds
所以我的问题是' ds'的连接语法。关系。当前错误提到添加remote(),但我没有让它工作。我也尝试过使用secondaryjoin而没有运气。查询' dq'工作和我最终能够通过在我的代码中使用过滤器来解决它 - 我仍然很好奇如何在可能的情况下构建反关系?
答案 0 :(得分:2)
我不是sqlalchemy专家,这是我的理解。
我认为sqlalchemy关系API中混淆的主要原因是参数primaryjoin
,secondary
,secondaryjoin
的真正意义。对我来说,他们是:
primaryjoin secondaryjoin(optional)
source -------------> secondary -------------------------> dest
(A) (D)
现在我们需要弄清楚中间部分应该是什么。尽管sqlalchemy中的自定义连接意外地复杂,但您确实需要了解您所要求的内容,即原始SQL。一种可能的解决方案是:
SELECT a.*, d.id
FROM a JOIN (b JOIN c ON c.b_id = b.id JOIN d ON d.id = c.d_id) /* secondary */
ON a.b_id = b.id /* primaryjoin */
WHERE a.id = 1;
在这种情况下,来源a
与“辅助”(b JOIN c .. JOIN d ..)
加入,并且D
没有辅助加入,因为它已经在secondary
中。我们有
ds1 = relationship(
'D',
primaryjoin='A.b_id == B.id',
secondary='join(B, C, B.id == C.b_id).join(D, C.d_id == D.id)',
viewonly=True, # almost always a better to add this
)
另一种解决方案可能是:
SELECT a.*, d.id
FROM a JOIN (b JOIN c ON c.b_id = b.id) /* secondary */
ON a.b_id = b.id /* primaryjoin */
JOIN d ON c.d_id = d.id /* secondaryjoin */
WHERE a.id = 1;
此处a
加入辅助(b JOIN c..)
,辅助加入d
加c.d_id = d.id
,因此:
ds2 = relationship(
'D',
primaryjoin='A.b_id == B.id',
secondary='join(B, C, B.id == C.b_id)',
secondaryjoin='C.d_id == D.id',
viewonly=True, # almost always a better to add this
)
经验法则是您在辅助路径中放置长连接路径,并将其链接到源和目标。
在性能方面,ds1
和ds2
导致查询计划比dq
略微简单,但我认为它们之间没有太大区别。规划师总是知道的更好。
以下是供您参考的更新代码。请注意您如何热切地加载与sess.query(A).options(joinedload('ds1'))
:
from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class A(Base):
__tablename__ = 'a'
id = Column(Integer, primary_key=True)
b_id = Column(Integer, ForeignKey('b.id'))
ds1 = relationship(
'D',
primaryjoin='A.b_id == B.id',
secondary='join(B, C, B.id == C.b_id).join(D, C.d_id == D.id)',
viewonly=True, # almost always a better to add this
)
ds2 = relationship(
'D',
secondary='join(B, C, B.id == C.b_id)',
primaryjoin='A.b_id == B.id',
secondaryjoin='C.d_id == D.id',
viewonly=True, # almost always a better to add this
)
def dq(self):
return sess.query(D).filter(and_(D.id == C.d_id,
C.b_id == B.id,
B.id == A.id,
A.id == self.id))
class B(Base):
__tablename__ = 'b'
id = Column(Integer, primary_key=True)
class C(Base):
__tablename__ = 'c'
b_id = Column(Integer, ForeignKey('b.id'), primary_key=True)
d_id = Column(Integer, ForeignKey('d.id'), primary_key=True)
class D(Base):
__tablename__ = 'd'
id = Column(Integer, primary_key=True)
def __repr__(self):
return str(self.id)
e = create_engine("sqlite://", echo=True)
Base.metadata.drop_all(e)
Base.metadata.create_all(e)
sess = Session(e)
sess.add(D(id=1))
sess.add(D(id=2))
sess.add(B(id=1))
sess.add(B(id=2))
sess.flush()
sess.add(C(b_id=1, d_id=1))
sess.add(C(b_id=1, d_id=2))
sess.add(A(id=1, b_id=1))
sess.add(A(id=2, b_id=2))
sess.commit()
def get_ids(ds):
return {d.id for d in ds}
a1 = sess.query(A).options(joinedload('ds1')).filter_by(id=1).first()
print('{} a1.ds1: {}'.format('=' * 30, a1.ds1))
assert get_ids(a1.dq()) == get_ids(a1.ds1)
a1 = sess.query(A).options(joinedload('ds2')).filter_by(id=1).first()
print('{} a1.ds2: {}'.format('=' * 30, a1.ds2))
assert get_ids(a1.dq()) == get_ids(a1.ds2)
a2 = sess.query(A).options(joinedload('ds2')).filter_by(id=2).first()
print('{} a2.ds1: {}; a2.ds2 {};'.format('=' * 30, a2.ds1, a2.ds2))
assert a2.ds1 == a2.ds2 == []
答案 1 :(得分:0)
对于您的查询,您需要将表格加在一起。即:
def db(self);
return sess.query(D).join(B).filter(...)
我不确定你是否可以进行多次加入,但我不明白为什么不这样做。可以在此处找到对对象关系查询的良好参考:
http://docs.sqlalchemy.org/en/rel_0_9/orm/tutorial.html#working-with-related-objects