Fixed #22397 -- Issues removing M2M field with explicit through model

Changed the migration autodetector to remove models last so that FK
and M2M fields will not be left as dangling references. Added a check
in the migration state renderer to error out in the presence of
dangling references instead of leaving them as strings. Fixed a bug
in the sqlite backend to handle the deletion of M2M fields with
"through" models properly (i.e., do nothing successfully).

Thanks to melinath for report, loic for tests and andrewgodwin and
charettes for assistance with architecture.

tests/migrations/test_operations.py

@@ -1,9 +1,12 @@
 import unittest
-from django.db import connection, models, migrations, router
+
+from django.db import connection, migrations, models, router
+from django.db.migrations.migration import Migration
+from django.db.migrations.state import ProjectState
 from django.db.models.fields import NOT_PROVIDED
 from django.db.transaction import atomic
 from django.db.utils import IntegrityError
-from django.db.migrations.state import ProjectState
+
 from .test_base import MigrationTestBase
 
 
@@ -15,15 +18,16 @@ class OperationTests(MigrationTestBase):
     """
 
     def apply_operations(self, app_label, project_state, operations):
-        new_state = project_state.clone()
-        for operation in operations:
-            operation.state_forwards(app_label, new_state)
-
-        # Set up the database
+        migration = Migration('name', app_label)
+        migration.operations = operations
         with connection.schema_editor() as editor:
-            for operation in operations:
-                operation.database_forwards(app_label, editor, project_state, new_state)
-        return new_state
+            return migration.apply(project_state, editor)
+
+    def unapply_operations(self, app_label, project_state, operations):
+        migration = Migration('name', app_label)
+        migration.operations = operations
+        with connection.schema_editor() as editor:
+            return migration.unapply(project_state, editor)
 
     def set_up_test_model(self, app_label, second_model=False, related_model=False, mti_model=False):
         """
@@ -396,6 +400,38 @@ class OperationTests(MigrationTestBase):
         Pony = new_apps.get_model("test_alflmm", "Pony")
         self.assertTrue(Pony._meta.get_field('stables').blank)
 
+    def test_remove_field_m2m(self):
+        project_state = self.set_up_test_model("test_rmflmm", second_model=True)
+
+        project_state = self.apply_operations("test_rmflmm", project_state, operations=[
+            migrations.AddField("Pony", "stables", models.ManyToManyField("Stable", related_name="ponies"))
+        ])
+        self.assertTableExists("test_rmflmm_pony_stables")
+
+        operations = [migrations.RemoveField("Pony", "stables")]
+        self.apply_operations("test_rmflmm", project_state, operations=operations)
+        self.assertTableNotExists("test_rmflmm_pony_stables")
+
+        # And test reversal
+        self.unapply_operations("test_rmflmm", project_state, operations=operations)
+        self.assertTableExists("test_rmflmm_pony_stables")
+
+    def test_remove_field_m2m_with_through(self):
+        project_state = self.set_up_test_model("test_rmflmmwt", second_model=True)
+
+        self.assertTableNotExists("test_rmflmmwt_ponystables")
+        project_state = self.apply_operations("test_rmflmmwt", project_state, operations=[
+            migrations.CreateModel("PonyStables", fields=[
+                ("pony", models.ForeignKey('test_rmflmmwt.Pony')),
+                ("stable", models.ForeignKey('test_rmflmmwt.Stable')),
+            ]),
+            migrations.AddField("Pony", "stables", models.ManyToManyField("Stable", related_name="ponies", through='test_rmflmmwt.PonyStables'))
+        ])
+        self.assertTableExists("test_rmflmmwt_ponystables")
+
+        operations = [migrations.RemoveField("Pony", "stables")]
+        self.apply_operations("test_rmflmmwt", project_state, operations=operations)
+
     def test_remove_field(self):
         """
         Tests the RemoveField operation.