The complications are that when the attribute is set in Django 1.10,
it no longer counts as a deferred attribute, and it is not retrieved from the database.
Naively updating __set__ to retrieve the value if it is deferred leads to infinite
recursion because accessing the attribute involves loading data from the database
and trying to set the attribute based on that value. This commit introduces
a somewhat hacky flag that records whether we're already trying to set
the attribute further up in the call stack.
This commit adds a collection of wrapper classes for tracking fields
while still using custom descriptors that may be present. This fixes
a bug where deferring a model field with a custom descriptor meant
that the descriptor was overridden in all subsequent queries.
If a tracker is defined on an inherited model, where the parent has a ForeignKey,
the tracker will now correctly determine that the field_map takes `fk` -> `fk_id`
References #83.
Instead of patching the save method of a tracked model class, we can use
a signal handler on post_save, which means we can still pickle our model
class.
Note we can't just listen for the signal from the class we have, but
instead listen for all post_save signals. This means we actually install
a new signal handler for each tracked model class, which fires on all
model save occurrences (and returns immediately if this handler doesn't care).
We probably could improve this to have a registry of tracked models, or
something, that allows us to just install one signal handler, and filter
according to membership.
The FieldTracker has_changed method no longer returns True for any input
when the instance is unsaved and no longer raises a FieldError for
fields after the first save. The original ModelTracker behavior is
maintained.
