Consider these models:
class Foo(Base):
__tablename__ = 'foo'
id = Column(Integer, primary_key=True, autoincrement=True)
bar_id = Column(Integer, ForeignKey("bar.id"), unique=True)
bar = relationship(lambda: Bar, back_populates="foo")
class Bar(Base):
__tablename__ = "bar"
id = Column(Integer, primary_key=True, autoincrement=True)
foo = relationship(Foo, back_populates="bar", uselist=False)
When loading foo.bar, I would expect foo.bar.foo to be set to foo without
an additional query to the database, but this doesn't appear to be the
case. Test code:
engine = create_engine("sqlite://", echo=True)
Base.metadata.create_all(bind=engine)
session = Session(bind=engine)
session.add(Foo(bar=Bar()))
session.flush()
session.expunge_all()
f = session.query(Foo).first()
b = f.bar
print("there shouldn't be anymore SQL queries")
assert b.foo is f
This results in the following output:
[...]
2016-03-17 18:04:46,858 INFO sqlalchemy.engine.base.Engine BEGIN (implicit)
2016-03-17 18:04:46,858 INFO sqlalchemy.engine.base.Engine INSERT INTO bar
DEFAULT VALUES
2016-03-17 18:04:46,858 INFO sqlalchemy.engine.base.Engine ()
2016-03-17 18:04:46,859 INFO sqlalchemy.engine.base.Engine INSERT INTO foo
(bar_id) VALUES (?)
2016-03-17 18:04:46,859 INFO sqlalchemy.engine.base.Engine (1,)
2016-03-17 18:04:46,860 INFO sqlalchemy.engine.base.Engine SELECT foo.id AS
foo_id, foo.bar_id AS foo_bar_id
FROM foo
LIMIT ? OFFSET ?
2016-03-17 18:04:46,860 INFO sqlalchemy.engine.base.Engine (1, 0)
2016-03-17 18:04:46,861 INFO sqlalchemy.engine.base.Engine SELECT bar.id AS
bar_id
FROM bar
WHERE bar.id = ?
2016-03-17 18:04:46,861 INFO sqlalchemy.engine.base.Engine (1,)
there shouldn't be anymore SQL queries
2016-03-17 18:04:46,861 INFO sqlalchemy.engine.base.Engine SELECT foo.id AS
foo_id, foo.bar_id AS foo_bar_id
FROM foo
WHERE ? = foo.bar_id
2016-03-17 18:04:46,861 INFO sqlalchemy.engine.base.Engine (1,)
2016-03-17 18:04:46,862 INFO sqlalchemy.engine.base.Engine ROLLBACK
I understand that when uselist=True there might be other items in the list
so you can't populate that list, but when uselist=False, especially when
there's a unique constraint ensuring that there can only ever be one entry,
you should be able to populate it. Am I missing something?
Jack
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