Allow callable in defaults argument for QuerySet.get_or_create

[Maxime Lorant]

It would be nice if get_or_create handles callable in the defaults parameter so the defaults value are not computed at each selection. Indeed, if your default values for insertion contains selection of related object or any long computation, it is done at each call, even if the object can be retrieved without insertion (and so the defaults parameter is not used).

In term of code, I would like to do something like this:

SomeObject.objects.get_or_create(
       key1=value1, 
       key2=value2,
       defaults={
             'key3': lambda: long_processing_to_get_default(),
             # or even
             'key3': lambda: OtherModel.objects.get(pk=request.some_value)
       }
)

For backward-compatibility and ease of writing, it should still accept values that are not callables. See the patch attached (unit tests are missing so it is not complete!) My patch is wrong in fact: it computes the value before the selection and it consider the dict as a whole callable instead of a single key of the dict... Anyway the implementation inside the except DoesNotExist should be easy :)