On Jul 29, 1:43 pm, Mike Conley <mconl...@gmail.com> wrote:
> I have a need to clone an instance of a mapped class and insert the new
> instance in my database with only minor changes to attributes. To do this I
> want to create a new instance of the class with all non-key attributes
> copied from the old instance. Primary keys and foreign keys will be
> populated at insert time via normal processes for any instance.
>
> Here is a function I wrote to clone the non-key attributes to a new
> instance. Does this make sense or might I stumble into an unseen abyss
> somewhere, or is there an easier way to do this? (I did not use copy.copy()
> because I only want the non-key attributes copied)
> None of the classes involve polymorphism which is one area I thought there
> might be trouble.
> All of our classes use the default declarative __init__(), so the statement
> new = cls() is valid in all cases.
>
> def clone_instance(old):
> cls = old.__class__
> tbl = cls.__table__
> new = cls()
> for c in tbl.c:
> if c.primary_key or c.foreign_keys:
> continue
> setattr(new, c.name, getattr(old,c.name))
> return new
>
> usage is something like this
>
> obj = session.query(SomeClass).get(someid)
> newobj = clone_instance(obj)
> newobj.someattribute = some different value
> session.add(newobj)
> session.commit()
>
> This seems to work for the simple cases I ran so far, just wondering if
> there are any hidden problems or simplifications.
I'd love to find a reasonable way of cloning an instance, even with
the caveat that one could only copy attributes that are no primary or
foreigh key related.
Still, I to make that code "work" w/newer versions of SA:
def clone_instance(old):
new = type(old)()
m = sa_orm.object_mapper(old)
for c in m.columns:
if c.primary_key or c.foreign_keys:
continue
setattr(new, c.name, getattr(old,c.name))
return new
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