Fix migration autodector to work correctly with custom deconstructed values

django/db/migrations/autodetector.py

@@ -64,6 +64,23 @@ 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):
+            """
+            Recursive deconstruction for a field and its arguments.
+            """
+            if not hasattr(obj, 'deconstruct'):
+                return obj
+            deconstructed = obj.deconstruct()
+            if field:
+                deconstructed = deconstructed[1:]
+            name, args, kwargs = deconstructed
+            return (
+                name,
+                [_deep_deconstruct(value, field=False) for value in args],
+                dict([(key, _deep_deconstruct(value, field=False))
+                      for key, value in kwargs.items()])
+            )
+
         def _rel_agnostic_fields_def(fields):
             """
             Return a definition of the fields that ignores field names and
@@ -71,7 +88,7 @@ class MigrationAutodetector(object):
             """
             fields_def = []
             for name, field in fields:
-                deconstruction = field.deconstruct()[1:]
+                deconstruction = _deep_deconstruct(field)
                 if field.rel and field.rel.to:
                     del deconstruction[2]['to']
                 fields_def.append(deconstruction)
@@ -255,11 +272,11 @@ class MigrationAutodetector(object):
             new_model_state = self.to_state.models[app_label, model_name]
             field = new_model_state.get_field_by_name(field_name)
             # Scan to see if this is actually a rename!
-            field_dec = field.deconstruct()[1:]
+            field_dec = _deep_deconstruct(field)
             found_rename = False
             for rem_app_label, rem_model_name, rem_field_name in (old_fields - new_fields):
                 if rem_app_label == app_label and rem_model_name == model_name:
-                    old_field_dec = old_model_state.get_field_by_name(rem_field_name).deconstruct()[1:]
+                    old_field_dec = _deep_deconstruct(old_model_state.get_field_by_name(rem_field_name))
                     if field.rel and field.rel.to and 'to' in old_field_dec[2]:
                         old_rel_to = old_field_dec[2]['to']
                         if old_rel_to in renamed_models_rel:
@@ -326,8 +343,8 @@ class MigrationAutodetector(object):
             old_model_state = self.from_state.models[app_label, old_model_name]
             new_model_state = self.to_state.models[app_label, model_name]
             old_field_name = renamed_fields.get((app_label, model_name, field_name), field_name)
-            old_field_dec = old_model_state.get_field_by_name(old_field_name).deconstruct()[1:]
-            new_field_dec = new_model_state.get_field_by_name(field_name).deconstruct()[1:]
+            old_field_dec = _deep_deconstruct(old_model_state.get_field_by_name(old_field_name))
+            new_field_dec = _deep_deconstruct(new_model_state.get_field_by_name(field_name))
             if old_field_dec != new_field_dec:
                 self.add_to_migration(
                     app_label,

tests/migrations/test_autodetector.py

@@ -7,6 +7,15 @@ from django.db.migrations.graph import MigrationGraph
 from django.db import models
 
 
+class DeconstructableObject(object):
+    """
+    A custom deconstructable object.
+    """
+
+    def deconstruct(self):
+        return self.__module__ + '.' + self.__class__.__name__, [], {}
+
+
 class AutodetectorTests(TestCase):
     """
     Tests the migration autodetector.
@@ -17,6 +26,8 @@ class AutodetectorTests(TestCase):
     author_name_longer = ModelState("testapp", "Author", [("id", models.AutoField(primary_key=True)), ("name", models.CharField(max_length=400))])
     author_name_renamed = ModelState("testapp", "Author", [("id", models.AutoField(primary_key=True)), ("names", models.CharField(max_length=200))])
     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_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))])
@@ -550,6 +561,17 @@ class AutodetectorTests(TestCase):
         self.assertEqual(action.__class__.__name__, "AddField")
         self.assertEqual(action.name, "name")
 
+    def test_custom_deconstructable(self):
+        """
+        Two instances which deconstruct to the same value aren't considered a
+        change.
+        """
+        before = self.make_project_state([self.author_name_deconstructable_1])
+        after = self.make_project_state([self.author_name_deconstructable_2])
+        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).