My question from stackoverflow:
http://stackoverflow.com/questions/20519910/subqueryload-does-not-populate-relation-if-backref-is-joined
Why CASE 4 (see output) does not provide related children. It looks like if
it happens because subqueryload does not populate relation if backref is
joinedload. But what logic is behind this? Is it a bug?
Please, note diffence between these lines:
children1 = relationship('Child1', back_populates='parent',
cascade='all', lazy='noload')
children2 = relationship('Child2', back_populates='parent',
cascade='all', lazy='noload')
parent = relationship('Parent', back_populates='children1',
cascade='all', lazy='joined')
parent = relationship('Parent', back_populates='children2',
cascade='all', lazy='subquery')
parent =
session3.query(Parent).options([joinedload('children1')]).get(1)
parent =
session4.query(Parent).options([joinedload('children2')]).get(1)
parent =
session5.query(Parent).options([subqueryload('children1')]).get(1)
parent =
session6.query(Parent).options([subqueryload('children2')]).get(1)
-------- CODE ---------
from sqlalchemy import Column, Integer, String, ForeignKey
from sqlalchemy import create_engine
from sqlalchemy.orm import relationship, sessionmaker, subqueryload,
joinedload
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class Parent(Base):
__tablename__ = 'parent'
id = Column(Integer, primary_key=True)
name = Column(String(20))
children1 = relationship('Child1', back_populates='parent',
cascade='all', lazy='noload')
children2 = relationship('Child2', back_populates='parent',
cascade='all', lazy='noload')
class Child1(Base):
__tablename__ = 'child1'
id = Column(Integer, primary_key=True)
name = Column(String(20))
parent_id = Column(Integer, ForeignKey('parent.id',
ondelete='CASCADE'))
parent = relationship('Parent', back_populates='children1',
cascade='all', lazy='joined')
def __repr__(self):
return repr((self.id, self.name, self.parent_id))
class Child2(Base):
__tablename__ = 'child2'
id = Column(Integer, primary_key=True)
name = Column(String(20))
parent_id = Column(Integer, ForeignKey('parent.id',
ondelete='CASCADE'))
parent = relationship('Parent', back_populates='children2',
cascade='all', lazy='subquery')
def __repr__(self):
return repr((self.id, self.name, self.parent_id))
engine = create_engine('sqlite:///:memory:')
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)
session1 = Session()
session1.add(Parent(id=1, name='parent'))
session1.add(Child1(id=1, name='child1_1', parent_id=1))
session1.add(Child1(id=2, name='child1_2', parent_id=1))
session1.add(Child2(id=1, name='child2_1', parent_id=1))
session1.add(Child2(id=2, name='child2_2', parent_id=1))
session1.commit()
session1.expunge_all()
session1.close()
session2 = Session()
parent = session2.query(Parent).get(1)
print 'CASE 1 (no options):', parent.id, parent.children1,
parent.children2
session2.close()
session3 = Session()
parent =
session3.query(Parent).options([joinedload('children1')]).get(1)
print 'CASE 2 (joined vs joinedload):', parent.id, parent.children1
session3.close()
session4 = Session()
parent =
session4.query(Parent).options([joinedload('children2')]).get(1)
print 'CASE 3 (subquery vs joinedload):', parent.id, parent.children2
session4.close()
session5 = Session()
parent =
session5.query(Parent).options([subqueryload('children1')]).get(1)
print 'CASE 4 (joined vs subqueryload):', parent.id, parent.children1
session5.close()
session6 = Session()
parent =
session6.query(Parent).options([subqueryload('children2')]).get(1)
print 'CASE 5 (subquery vs subqueryload):', parent.id, parent.children2
session6.close()
Output
CASE 1 (no options): 1 [] []
CASE 2 (joined vs joinedload): 1 [(1, u'child1_1', 1), (2, u'child1_2',
1)]
CASE 3 (subquery vs joinedload): 1 [(1, u'child2_1', 1), (2,
u'child2_2', 1)]
CASE 4 (joined vs subqueryload): 1 []
CASE 5 (subquery vs subqueryload): 1 [(1, u'child2_1', 1), (2,
u'child2_2', 1)]
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