[3.1.x] Fixed #31941 -- Corrected FileField.deconstruct() with a callable storage.

Backport of 2d42e23b6d from master

AUTHORS

@@ -144,6 +144,7 @@ answer newbie questions, and generally made Django that much better:
     Brian Beck <http://blog.brianbeck.com/>
     Brian Fabian Crain <http://www.bfc.do/>
     Brian Harring <ferringb@gmail.com>
+    Brian Helba <brian.helba@kitware.com>
     Brian Ray <http://brianray.chipy.org/>
     Brian Rosner <brosner@gmail.com>
     Bruce Kroeze <https://coderseye.com/>

django/db/models/fields/files.py

@@ -235,6 +235,8 @@ class FileField(Field):
 
         self.storage = storage or default_storage
         if callable(self.storage):
+            # Hold a reference to the callable for deconstruct().
+            self._storage_callable = self.storage
             self.storage = self.storage()
             if not isinstance(self.storage, Storage):
                 raise TypeError(
@@ -285,7 +287,7 @@ class FileField(Field):
             del kwargs["max_length"]
         kwargs['upload_to'] = self.upload_to
         if self.storage is not default_storage:
-            kwargs['storage'] = self.storage
+            kwargs['storage'] = getattr(self, '_storage_callable', self.storage)
         return name, path, args, kwargs
 
     def get_internal_type(self):

docs/releases/3.1.2.txt

@@ -9,4 +9,5 @@ Django 3.1.2 fixes several bugs in 3.1.1.
 Bugfixes
 ========
 
-* ...
+* Fixed a bug in Django 3.1 where ``FileField`` instances with a callable
+  storage were not correctly deconstructed (:ticket:`31941`).

tests/file_storage/tests.py

@@ -29,7 +29,9 @@ from django.test.utils import requires_tz_support
 from django.urls import NoReverseMatch, reverse_lazy
 from django.utils import timezone
 
-from .models import Storage, temp_storage, temp_storage_location
+from .models import (
+    Storage, callable_storage, temp_storage, temp_storage_location,
+)
 
 FILE_SUFFIX_REGEX = '[A-Za-z0-9]{7}'
 
@@ -912,6 +914,15 @@ class FieldCallableFileStorageTests(SimpleTestCase):
         self.assertEqual(obj.storage_callable.storage.location, temp_storage_location)
         self.assertIsInstance(obj.storage_callable_class.storage, BaseStorage)
 
+    def test_deconstruction(self):
+        """
+        Deconstructing gives the original callable, not the evaluated value.
+        """
+        obj = Storage()
+        *_, kwargs = obj._meta.get_field('storage_callable').deconstruct()
+        storage = kwargs['storage']
+        self.assertIs(storage, callable_storage)
+
 
 # Tests for a race condition on file saving (#4948).
 # This is written in such a way that it'll always pass on platforms