Fixed #36438 -- Made MigrationAutodetector remove generated fields before their base fields.

Thanks to Colton Saska for the report and to Simon Charette for the review.

django/db/migrations/autodetector.py

@@ -1221,9 +1221,9 @@ class MigrationAutodetector:
                 model_name=model_name,
                 name=field_name,
             ),
-            # We might need to depend on the removal of an
+            # Include dependencies such as order_with_respect_to, constraints,
-            # order_with_respect_to or index/constraint/unique_together
+            # and any generated fields that may depend on this field. These
-            # operation; this is safely ignored if there isn't one
+            # are safely ignored if not present.
             dependencies=[
                 OperationDependency(
                     app_label,
@@ -1243,6 +1243,9 @@ class MigrationAutodetector:
                     field_name,
                     OperationDependency.Type.REMOVE_INDEX_OR_CONSTRAINT,
                 ),
+                *self._get_generated_field_dependencies_for_removed_field(
+                    app_label, model_name, field_name
+                ),
             ],
         )
 
@@ -1698,6 +1701,27 @@ class MigrationAutodetector:
                 )
         return dependencies
 
+    def _get_generated_field_dependencies_for_removed_field(
+        self, app_label, model_name, field_name
+    ):
+        dependencies = []
+        model_state = self.from_state.models[app_label, model_name]
+        generated_fields = (f for f in model_state.fields.values() if f.generated)
+        for field in generated_fields:
+            if any(
+                field_name == name
+                for name, *_ in models.Model._get_expr_references(field.expression)
+            ):
+                dependencies.append(
+                    OperationDependency(
+                        app_label,
+                        model_name,
+                        field.name,
+                        OperationDependency.Type.REMOVE,
+                    )
+                )
+        return dependencies
+
     def _get_dependencies_for_model(self, app_label, model_name):
         """Return foreign key dependencies of the given model."""
         dependencies = []

tests/migrations/test_autodetector.py

@@ -13,7 +13,7 @@ from django.db.migrations.graph import MigrationGraph
 from django.db.migrations.loader import MigrationLoader
 from django.db.migrations.questioner import MigrationQuestioner
 from django.db.migrations.state import ModelState, ProjectState
-from django.db.models.functions import Concat, Lower
+from django.db.models.functions import Concat, Lower, Upper
 from django.test import SimpleTestCase, TestCase, override_settings
 from django.test.utils import isolate_lru_cache
 
@@ -1454,6 +1454,100 @@ class AutodetectorTests(BaseAutodetectorTests):
         self.assertOperationTypes(changes, "testapp", 0, ["RemoveField"])
         self.assertOperationAttributes(changes, "testapp", 0, 0, name="name")
 
+    def test_remove_generated_field_before_its_base_field(self):
+        initial_state = [
+            ModelState(
+                "testapp",
+                "Author",
+                [
+                    ("name", models.CharField(max_length=20)),
+                    (
+                        "upper_name",
+                        models.GeneratedField(
+                            expression=Upper("name"),
+                            db_persist=True,
+                            output_field=models.CharField(),
+                        ),
+                    ),
+                ],
+            ),
+        ]
+        updated_state = [ModelState("testapp", "Author", [])]
+        changes = self.get_changes(initial_state, updated_state)
+        self.assertNumberMigrations(changes, "testapp", 1)
+        self.assertOperationTypes(changes, "testapp", 0, ["RemoveField", "RemoveField"])
+        self.assertOperationAttributes(changes, "testapp", 0, 0, name="upper_name")
+        self.assertOperationAttributes(changes, "testapp", 0, 1, name="name")
+
+    def test_remove_generated_field_before_multiple_base_fields(self):
+        initial_state = [
+            ModelState(
+                "testapp",
+                "Author",
+                [
+                    ("first_name", models.CharField(max_length=20)),
+                    ("last_name", models.CharField(max_length=20)),
+                    (
+                        "full_name",
+                        models.GeneratedField(
+                            expression=Concat("first_name", "last_name"),
+                            db_persist=True,
+                            output_field=models.CharField(),
+                        ),
+                    ),
+                ],
+            ),
+        ]
+        updated_state = [ModelState("testapp", "Author", [])]
+        changes = self.get_changes(initial_state, updated_state)
+        self.assertNumberMigrations(changes, "testapp", 1)
+        self.assertOperationTypes(
+            changes, "testapp", 0, ["RemoveField", "RemoveField", "RemoveField"]
+        )
+        self.assertOperationAttributes(changes, "testapp", 0, 0, name="full_name")
+        self.assertOperationAttributes(changes, "testapp", 0, 1, name="first_name")
+        self.assertOperationAttributes(changes, "testapp", 0, 2, name="last_name")
+
+    def test_remove_generated_field_and_one_of_multiple_base_fields(self):
+        initial_state = [
+            ModelState(
+                "testapp",
+                "Author",
+                [
+                    ("first_name", models.CharField(max_length=20)),
+                    ("last_name", models.CharField(max_length=20)),
+                    (
+                        "full_name",
+                        models.GeneratedField(
+                            expression=Concat("first_name", "last_name"),
+                            db_persist=True,
+                            output_field=models.CharField(),
+                        ),
+                    ),
+                ],
+            ),
+        ]
+        # Only remove full_name and first_name.
+        updated_state = [
+            ModelState(
+                "testapp",
+                "Author",
+                [
+                    ("last_name", models.CharField(max_length=20)),
+                ],
+            ),
+        ]
+        changes = self.get_changes(initial_state, updated_state)
+        self.assertNumberMigrations(changes, "testapp", 1)
+        self.assertOperationTypes(
+            changes,
+            "testapp",
+            0,
+            ["RemoveField", "RemoveField"],
+        )
+        self.assertOperationAttributes(changes, "testapp", 0, 0, name="full_name")
+        self.assertOperationAttributes(changes, "testapp", 0, 1, name="first_name")
+
     def test_alter_field(self):
         """Tests autodetection of new fields."""
         changes = self.get_changes([self.author_name], [self.author_name_longer])