Added test for removing through model from ManyToManyField.

tests/schema/tests.py

@@ -1894,6 +1894,41 @@ class SchemaTests(TransactionTestCase):
     def test_m2m_create_through_inherited(self):
         self._test_m2m_create_through(InheritedManyToManyField)
 
+    def test_m2m_through_remove(self):
+        class LocalAuthorNoteThrough(Model):
+            book = ForeignKey("schema.Author", CASCADE)
+            tag = ForeignKey("self", CASCADE)
+
+            class Meta:
+                app_label = "schema"
+                apps = new_apps
+
+        class LocalNoteWithM2MThrough(Model):
+            authors = ManyToManyField("schema.Author", through=LocalAuthorNoteThrough)
+
+            class Meta:
+                app_label = "schema"
+                apps = new_apps
+
+        self.local_models = [LocalAuthorNoteThrough, LocalNoteWithM2MThrough]
+        # Create the tables.
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+            editor.create_model(LocalAuthorNoteThrough)
+            editor.create_model(LocalNoteWithM2MThrough)
+        # Remove the through parameter.
+        old_field = LocalNoteWithM2MThrough._meta.get_field("authors")
+        new_field = ManyToManyField("Author")
+        new_field.set_attributes_from_name("authors")
+        msg = (
+            f"Cannot alter field {old_field} into {new_field} - they are not "
+            f"compatible types (you cannot alter to or from M2M fields, or add or "
+            f"remove through= on M2M fields)"
+        )
+        with connection.schema_editor() as editor:
+            with self.assertRaisesMessage(ValueError, msg):
+                editor.alter_field(LocalNoteWithM2MThrough, old_field, new_field)
+
     def _test_m2m(self, M2MFieldClass):
         """
         Tests adding/removing M2M fields on models