[1.8.x] Refs #24225 -- Added failing test case for removing a previously added field in migrations

When a related field is deleted, the related model must be updated. As
unchanged models are shared in migration states, the related model must
be re-rendered so that the change applies to a new copy of the related
model.

Thanks Henrik Heimbuerger for the report.

Backport of 58d0dd9260 from master

tests/migrations/test_state.py

@@ -1,6 +1,6 @@
 from django.apps.registry import Apps
 from django.db import models
-from django.db.migrations.operations import RemoveField
+from django.db.migrations.operations import DeleteModel, RemoveField
 from django.db.migrations.state import (
     InvalidBasesError, ModelState, ProjectState,
 )
@@ -366,6 +366,53 @@ class StateTests(TestCase):
         project_state.add_model(ModelState.from_model(B))
         self.assertEqual(len(project_state.apps.get_models()), 2)
 
+    def test_remove_relations(self):
+        """
+        #24225 - Tests that relations between models are updated while
+        remaining the relations and references for models of an old state.
+        """
+        class A(models.Model):
+            class Meta:
+                app_label = "something"
+
+        class B(models.Model):
+            to_a = models.ForeignKey(A)
+
+            class Meta:
+                app_label = "something"
+
+        def get_model_a(state):
+            return [mod for mod in state.apps.get_models() if mod._meta.model_name == 'a'][0]
+
+        project_state = ProjectState()
+        project_state.add_model(ModelState.from_model(A))
+        project_state.add_model(ModelState.from_model(B))
+        self.assertEqual(len(get_model_a(project_state)._meta.related_objects), 1)
+        old_state = project_state.clone()
+
+        operation = RemoveField("b", "to_a")
+        operation.state_forwards("something", project_state)
+        # Tests that model from old_state still has the relation
+        model_a_old = get_model_a(old_state)
+        model_a_new = get_model_a(project_state)
+        self.assertIsNot(model_a_old, model_a_new)
+        self.assertEqual(len(model_a_old._meta.related_objects), 1)
+        self.assertEqual(len(model_a_new._meta.related_objects), 0)
+
+        # Same test for deleted model
+        project_state = ProjectState()
+        project_state.add_model(ModelState.from_model(A))
+        project_state.add_model(ModelState.from_model(B))
+        old_state = project_state.clone()
+
+        operation = DeleteModel("b")
+        operation.state_forwards("something", project_state)
+        model_a_old = get_model_a(old_state)
+        model_a_new = get_model_a(project_state)
+        self.assertIsNot(model_a_old, model_a_new)
+        self.assertEqual(len(model_a_old._meta.related_objects), 1)
+        self.assertEqual(len(model_a_new._meta.related_objects), 0)
+
     def test_equality(self):
         """
         Tests that == and != are implemented correctly.
@@ -384,11 +431,13 @@ class StateTests(TestCase):
             {},
             None,
         ))
+        project_state.apps  # Fill the apps cached property
         other_state = project_state.clone()
         self.assertEqual(project_state, project_state)
         self.assertEqual(project_state, other_state)
         self.assertEqual(project_state != project_state, False)
         self.assertEqual(project_state != other_state, False)
+        self.assertNotEqual(project_state.apps, other_state.apps)
 
         # Make a very small change (max_len 99) and see if that affects it
         project_state = ProjectState()