Thanks for replying. Isn't it the case when I just provide a primary key
for the source object, the attribute shoul be marked expired on the target
instance? In my example I only give the object u a primary key value, so
the newly created object should be an object that only has a primary key
value and "wipe out" other attributes originally for that row in the table.
However the result is that row stays unchanged after all my instructions.
Is there something I missed here?
On Feb 19, 2014 6:47 AM, "Michael Bayer" <mike...@zzzcomputing.com> wrote:
>
> On Feb 19, 2014, at 6:08 AM, Bao Niu <niuba...@gmail.com> wrote:
>
> I thought I understood the behaviour of session.merge() from reading the
> documentation, but I'm wrong. Here is an example. I have codes below,
>
> class User(Base):
>> __tablename__ = 'user'
>> id = Column(Integer, primary_key=True)
>> name = Column(String(50), nullable=False)
>> addresses = relationship("Address", backref="user")
>
> This class has a corresponding table 'user' which has already been
> populated with one row, with its primary key equal to 1.
> Now I run the following instructions from python -i:
>
>> >>> u = User(id=1)
>> >>> session.merge(u)
>> >>> session.commit()
>
> I expected that there should be a sqlalchemy.orm.exc.FlushError, because
> both object u (which has been merged) and the row which has been already in
> the table have their primary key equal to 1. However, nothing happened.
> Sqlalchemy just ignored that merged u!
> If I do these instructions:
>
>> >>> session.add(u)
>> >>> session.commit()
>
> Then I got a sqlalchemy.orm.exc.FlushError, which was expected.
>
> So, session.merge() does not include .add()? From the documentation it
> seems that it does.
>
>
> So first is that the object you pass to merge() is *never* itself added to
> the session. The object you pass in is unmodified. The return value of
> merge() is the object that is actually in the Session. This object will
> either have been newly created, in which case it was added using add(), or
> it was loaded from the database. But that object is never the one you
> passed in.
>
> So in your test if primary key "1" already exists, then merge() returned
> the User object it had for that primary key.
>
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