Fixed #28350 -- Fixed UnboundLocalError crash in RenameField with nonexistent field.

Thanks Tim for the review.

django/db/migrations/operations/fields.py

@@ -1,3 +1,4 @@
+from django.core.exceptions import FieldDoesNotExist
 from django.db.models.fields import NOT_PROVIDED
 from django.utils.functional import cached_property
 
@@ -261,25 +262,27 @@ class RenameField(FieldOperation):
         )
 
     def state_forwards(self, app_label, state):
+        model_state = state.models[app_label, self.model_name_lower]
         # Rename the field
-        state.models[app_label, self.model_name_lower].fields = [
-            (self.new_name if n == self.old_name else n, f)
-            for n, f in state.models[app_label, self.model_name_lower].fields
-        ]
+        fields = model_state.fields
+        for index, (name, field) in enumerate(fields):
+            if name == self.old_name:
+                fields[index] = (self.new_name, field)
+                # Delay rendering of relationships if it's not a relational field.
+                delay = not field.is_relation
+                break
+        else:
+            raise FieldDoesNotExist(
+                "%s.%s has no field named '%s'" % (app_label, self.model_name, self.old_name)
+            )
         # Fix index/unique_together to refer to the new field
-        options = state.models[app_label, self.model_name_lower].options
+        options = model_state.options
         for option in ('index_together', 'unique_together'):
             if option in options:
                 options[option] = [
                     [self.new_name if n == self.old_name else n for n in together]
                     for together in options[option]
                 ]
-        for n, f in state.models[app_label, self.model_name_lower].fields:
-            if n == self.new_name:
-                field = f
-                break
-        # Delay rendering of relationships if it's not a relational field
-        delay = not field.is_relation
         state.reload_model(app_label, self.model_name_lower, delay=delay)
 
     def database_forwards(self, app_label, schema_editor, from_state, to_state):

docs/releases/1.11.3.txt

@@ -54,3 +54,6 @@ Bugfixes
 
 * Prevented a primary key alteration from adding a foreign key constraint if
   ``db_constraint=False`` (:ticket:`28298`).
+
+* Fixed ``UnboundLocalError`` crash in ``RenameField`` with nonexistent field
+  (:ticket:`28350`).

tests/migrations/test_operations.py

@@ -1,5 +1,6 @@
 import unittest
 
+from django.core.exceptions import FieldDoesNotExist
 from django.db import connection, migrations, models, transaction
 from django.db.migrations.migration import Migration
 from django.db.migrations.operations import CreateModel
@@ -1368,6 +1369,12 @@ class OperationTests(OperationTestBase):
         self.assertEqual(definition[1], [])
         self.assertEqual(definition[2], {'model_name': "Pony", 'old_name': "pink", 'new_name': "blue"})
 
+    def test_rename_missing_field(self):
+        state = ProjectState()
+        state.add_model(ModelState('app', 'model', []))
+        with self.assertRaisesMessage(FieldDoesNotExist, "app.model has no field named 'field'"):
+            migrations.RenameField('model', 'field', 'new_field').state_forwards('app', state)
+
     def test_alter_unique_together(self):
         """
         Tests the AlterUniqueTogether operation.