Field._get_default() prevents model fields from being pickled

[Adam Alton]

The changes introduced in ​d2a26c1a90e837777dabdf3d67ceec4d2a70fb86 mean that in cases where a model field has a default and the default is not a callable, the field instance cannot be pickled.

The offending line is:

  return lambda: self.default

This problem only exists in Django version >= 1.11.

The reason for that lambda is because the code has been refactored so that all the logic for getting the default value has been moved into the _get_default method, which allows it to be cached with the @cached_property decorator. And then the get_default method expects the value returned from _get_default to always be a callable. That requirement for the value to always be a callable is the reason for wrapping the value with lambda in the case where it's not already a callable.

My proposed solution is to simply move the if callable() check back out into the get_default method. This would mean that we lose a very minor bit of efficiency because we have to run that one if statement each time that get_default is called, but the rest of the logic would remain inside the cached _get_default method.

I have made a commit here which contains my proposed fix and a test to prevent regression: ​https://github.com/adamalton/django/commit/ee6aafacfd01f2603943e8c4183805fd4a298355