From 114da2d0455594bae0bbda8f2b1d46c5ec854956 Mon Sep 17 00:00:00 2001
From: Nan Liu <45443761+nanlliu@users.noreply.github.com>
Date: Sun, 24 May 2020 08:59:46 +0800
Subject: [PATCH] [3.1.x] Fixed #31416 -- Made autodetector find dependencies
 for MTI model creation on base fields removal.

Removing a base field must take place before adding a new inherited
model that has a field with the same name.

Backport of 33c365781abbcc1b21a31b31d95d344a174df0d5 from master
---
 django/db/migrations/autodetector.py  | 10 ++++++++++
 tests/migrations/test_autodetector.py | 25 +++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/django/db/migrations/autodetector.py b/django/db/migrations/autodetector.py
index 30616c0172b..85c30138975 100644
--- a/django/db/migrations/autodetector.py
+++ b/django/db/migrations/autodetector.py
@@ -563,6 +563,16 @@ class MigrationAutodetector:
                 if isinstance(base, str) and "." in base:
                     base_app_label, base_name = base.split(".", 1)
                     dependencies.append((base_app_label, base_name, None, True))
+                    # Depend on the removal of base fields if the new model has
+                    # a field with the same name.
+                    old_base_model_state = self.from_state.models.get((base_app_label, base_name))
+                    new_base_model_state = self.to_state.models.get((base_app_label, base_name))
+                    if old_base_model_state and new_base_model_state:
+                        removed_base_fields = set(old_base_model_state.fields).difference(
+                            new_base_model_state.fields,
+                        ).intersection(model_state.fields)
+                        for removed_base_field in removed_base_fields:
+                            dependencies.append((base_app_label, base_name, removed_base_field, False))
             # Depend on the other end of the primary key if it's a relation
             if primary_key_rel:
                 dependencies.append((
diff --git a/tests/migrations/test_autodetector.py b/tests/migrations/test_autodetector.py
index daff57fc2cf..10af72561b4 100644
--- a/tests/migrations/test_autodetector.py
+++ b/tests/migrations/test_autodetector.py
@@ -2454,3 +2454,28 @@ class AutodetectorTests(TestCase):
         self.assertNumberMigrations(changes, 'app', 1)
         self.assertOperationTypes(changes, 'app', 0, ['DeleteModel'])
         self.assertOperationAttributes(changes, 'app', 0, 0, name='Dog')
+
+    def test_add_model_with_field_removed_from_base_model(self):
+        """
+        Removing a base field takes place before adding a new inherited model
+        that has a field with the same name.
+        """
+        before = [
+            ModelState('app', 'readable', [
+                ('id', models.AutoField(primary_key=True)),
+                ('title', models.CharField(max_length=200)),
+            ]),
+        ]
+        after = [
+            ModelState('app', 'readable', [
+                ('id', models.AutoField(primary_key=True)),
+            ]),
+            ModelState('app', 'book', [
+                ('title', models.CharField(max_length=200)),
+            ], bases=('app.readable',)),
+        ]
+        changes = self.get_changes(before, after)
+        self.assertNumberMigrations(changes, 'app', 1)
+        self.assertOperationTypes(changes, 'app', 0, ['RemoveField', 'CreateModel'])
+        self.assertOperationAttributes(changes, 'app', 0, 0, name='title', model_name='readable')
+        self.assertOperationAttributes(changes, 'app', 0, 1, name='book')