[fix]: Prevent FallbackMixin from generating spurious migrations

The deconstruct() method was serializing the fallback kwarg into Django migration files. This caused new migrations to be generated whenever the fallback default value changed in settings, even though no actual database schema change had occurred.

The fix removes fallback from deconstruct() so Django no longer tracks it as part of the field migration state. fallback is also made optional in __init__ (defaulting to None) so existing migrations that omit the kwarg remain valid.

Fixes: #1231

Author: shivsubh

openwisp_utils/fields.py

@@ -48,14 +48,13 @@ class FallbackMixin(object):
     """
 
     def __init__(self, *args, **kwargs):
-        self.fallback = kwargs.pop("fallback")
+        self.fallback = kwargs.pop("fallback", None)
         opts = dict(blank=True, null=True, default=None)
         opts.update(kwargs)
         super().__init__(*args, **opts)
 
     def deconstruct(self):
         name, path, args, kwargs = super().deconstruct()
-        kwargs["fallback"] = self.fallback
         return (name, path, args, kwargs)
 
     def from_db_value(self, value, expression, connection):