Я пытаюсь создать отношения, которые охватывают 3 таблицы, но я не могу понять синтаксис.
У меня есть 3 таблицы TableA, TableB и TableC, и отношение, которое я пытаюсь моделировать:
TableA.my_relationship = relationship(
'TableC',
primaryjoin='and_(TableA.fk == TableB.pk, TableB.fk == TableC.pk)',
viewonly=True
)
так что в экземпляре TableA я могу сделать instance_of_a.my_relationship, чтобы получить запись TableC, которая косвенно связана с instance_of_a
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'))
# method one - put everything into primaryjoin.
# will work for simple lazy loads but for eager loads the ORM
# will fail to build up the FROM to correctly include B
cs = relationship("C",
# C.id is "foreign" because there can be many C.ids for one A.id
# B.id is "remote", it sort of means "this is where the stuff
# starts that not directly part of the A side"
primaryjoin="and_(A.b_id == remote(B.id), foreign(C.id) == B.c_id)",
viewonly=True)
# method two - split out the middle table into "secondary".
# note 'b' is the table name in metadata.
# this method will work better, as the ORM can also handle
# eager loading with this one.
c_via_secondary = relationship("C", secondary="b",
primaryjoin="A.b_id == B.id", secondaryjoin="C.id == B.c_id",
viewonly=True)
class B(Base):
__tablename__ = 'b'
id = Column(Integer, primary_key=True)
c_id = Column(Integer, ForeignKey('c.id'))
class C(Base):
__tablename__ = 'c'
id = Column(Integer, primary_key=True)
e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)
sess = Session(e)
sess.add(C(id=1))
sess.flush()
sess.add(B(id=1, c_id=1))
sess.flush()
sess.add(A(b_id=1))
sess.flush()
a1 = sess.query(A).first()
print(a1.cs)
print(a1.c_via_secondary)