A persistent object doesn't populate an unloaded backref on a forward set
event. This is for efficiency so that when you do something like:
for b in Session.query(B):
b.a = some_a
it doesn't spend time loading the "bs" collection of "some_a", which if you had
a lot of different "some_a" would take a lot of time. The other direction:
for a in Session.query(A):
a.bs.append(some_b)
if you were to access "some_b.a", the lookup is from the identity map since
some_b is present. There is a step that ensures that the "change" from the
backref is present in the "history" of the other side, but this is done in such
a way as to not force a collection or reference load.
I frequently forget the details of behaviors like these since 90% of them have
been nailed down years ago, so if you try the following test case, you'll see
no SQL is emitted after "2.----".
Also I have to run out so I may be forgetting some other details about this,
I'll try to take a second look later.
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)
bs = relationship("B", backref="a")
class B(Base):
__tablename__ = 'b'
id = Column(Integer, primary_key=True)
a_id = Column(Integer, ForeignKey('a.id'))
e = create_engine('sqlite://', echo=True)
Base.metadata.create_all(e)
s = Session(e)
s.add(A(id=1, bs=[B(id=1), B(id=2)]))
s.commit()
s.close()
a = A(id=1, bs=[B(id=1), B(id=2)])
print "1. -----------------------------------------"
a2 = s.merge(a)
print "2. -----------------------------------------"
for b in a2.bs:
assert b.a is a2
On Jul 6, 2011, at 4:24 PM, Kent wrote:
> If I merge() an object with a collection property, the backref's are
> not set as they would be if I had assigned the collection to the
> object.
>
> I expected that this should occur. Is there rationale for not setting
> backref's or would it be possible to make this change?
>
> Thanks,
> Kent
>
> --
> You received this message because you are subscribed to the Google Groups
> "sqlalchemy" group.
> To post to this group, send email to sqlalchemy@googlegroups.com.
> To unsubscribe from this group, send email to
> sqlalchemy+unsubscr...@googlegroups.com.
> For more options, visit this group at
> http://groups.google.com/group/sqlalchemy?hl=en.
>
--
You received this message because you are subscribed to the Google Groups
"sqlalchemy" group.
To post to this group, send email to sqlalchemy@googlegroups.com.
To unsubscribe from this group, send email to
sqlalchemy+unsubscr...@googlegroups.com.
For more options, visit this group at
http://groups.google.com/group/sqlalchemy?hl=en.
