Made nested deconstruction support both forms of deconstruct()

Nested deconstruction should (silently) handle Field.deconstruct() as
well as other arbitrary deconstructable objects. This allows having a
field in the deconstruction of another field.

django/db/migrations/autodetector.py

@@ -67,20 +67,21 @@ class MigrationAutodetector(object):
             if not model._meta.proxy and model._meta.managed and al not in self.to_state.real_apps:
                 new_model_keys.append((al, mn))
 
-        def _deep_deconstruct(obj, field=True):
+        def _deep_deconstruct(obj):
             """
             Recursive deconstruction for a field and its arguments.
             """
             if not hasattr(obj, 'deconstruct'):
                 return obj
             deconstructed = obj.deconstruct()
-            if field:
+            if isinstance(obj, models.Field):
+                # we have a field which also returns a name
                 deconstructed = deconstructed[1:]
             name, args, kwargs = deconstructed
             return (
                 name,
-                [_deep_deconstruct(value, field=False) for value in args],
+                [_deep_deconstruct(value) for value in args],
-                dict([(key, _deep_deconstruct(value, field=False))
+                dict([(key, _deep_deconstruct(value))
                       for key, value in kwargs.items()])
             )
 

tests/migrations/test_autodetector.py

@@ -28,6 +28,8 @@ class AutodetectorTests(TestCase):
     author_name_default = ModelState("testapp", "Author", [("id", models.AutoField(primary_key=True)), ("name", models.CharField(max_length=200, default='Ada Lovelace'))])
     author_name_deconstructable_1 = ModelState("testapp", "Author", [("id", models.AutoField(primary_key=True)), ("name", models.CharField(max_length=200, default=DeconstructableObject()))])
     author_name_deconstructable_2 = ModelState("testapp", "Author", [("id", models.AutoField(primary_key=True)), ("name", models.CharField(max_length=200, default=DeconstructableObject()))])
+    author_name_deconstructable_3 = ModelState("testapp", "Author", [("id", models.AutoField(primary_key=True)), ("name", models.CharField(max_length=200, default=models.IntegerField()))])
+    author_name_deconstructable_4 = ModelState("testapp", "Author", [("id", models.AutoField(primary_key=True)), ("name", models.CharField(max_length=200, default=models.IntegerField()))])
     author_with_book = ModelState("testapp", "Author", [("id", models.AutoField(primary_key=True)), ("name", models.CharField(max_length=200)), ("book", models.ForeignKey("otherapp.Book"))])
     author_renamed_with_book = ModelState("testapp", "Writer", [("id", models.AutoField(primary_key=True)), ("name", models.CharField(max_length=200)), ("book", models.ForeignKey("otherapp.Book"))])
     author_with_publisher_string = ModelState("testapp", "Author", [("id", models.AutoField(primary_key=True)), ("name", models.CharField(max_length=200)), ("publisher_name", models.CharField(max_length=200))])
@@ -576,6 +578,16 @@ class AutodetectorTests(TestCase):
         changes = autodetector._detect_changes()
         self.assertEqual(changes, {})
 
+    def test_deconstruct_field_kwarg(self):
+        """
+        Field instances are handled correctly by nested deconstruction.
+        """
+        before = self.make_project_state([self.author_name_deconstructable_3])
+        after = self.make_project_state([self.author_name_deconstructable_4])
+        autodetector = MigrationAutodetector(before, after)
+        changes = autodetector._detect_changes()
+        self.assertEqual(changes, {})
+
     def test_replace_string_with_foreignkey(self):
         """
         Adding an FK in the same "spot" as a deleted CharField should work. (#22300).