SQLAlchemy instruments classes using an event-based system that
intercepts all setattribute and collection mutation operations, and
logs these events as things to be reconciled when the Session is
flushed. Additionally, the __dict__ of the object itself is
referenced by the InstanceState object which is SQLAlchemy's per-
object manager. Thirdly, the InstanceState object itself is placed
inside of the __dict__ of every object instance, and this cannot be
shared among multiple objects. So for these three reasons and
probably more, using the old Python trick of replacing __dict__ will
lead to poor results.
To populate the state of an object with that of another, use a
setattr() based approach:
for k in dir(object1):
setattr(object2, k, getattr(object1, k))
On Dec 3, 2008, at 3:47 PM, [EMAIL PROTECTED] wrote:
> The reason I'm doing so, is to solve the following problem: I have an
> object that is compounded from the fields - obj_id, num1, num2, num3.
> obj_id is my primary key. I want to create a save method for the
> object's class, that will do the following:
> If the object is in the session, save it.
> Else, if there is another object in the db with the same num1 and
> num2, use the object in the db instead of the current one, and warn
> the user if num3 is different.
> So it's quite like merge, but not necessarily on the primary key. Now,
> I want that "use the object in the db" wouldn't be by "returning" the
> object after the merge (original object if didn't exist, and db_object
> if existed), but really "replacing" it (self.dict =
> existing_object.dict), so one can use that object afterwards, without
> being confused.
the most straightforward way to accomplish this would be a custom
__new__ method, so that when you say:
x = MyObject(num1=x, num2=y)
the __new__() would do the session work to find the old object and
return that instead, if present. There is a usage recipe on the Wiki
that does something like this, except its 0.4 specific, and uses a
metaclass in combination with a homegrown cache - I wrote it years ago
and it can be done much more simply. Just use __new__().
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