Fix nullability changing code

django/db/backends/schema.py

@@ -495,7 +495,7 @@ def alter_field(self, model, old_field, new_field, strict=False):
                 ))
             else:
                 actions.append((
-                    self.sql_alter_column_null % {
+                    self.sql_alter_column_not_null % {
                         "column": self.quote_name(new_field.column),
                         "type": new_type,
                     },

django/db/backends/sqlite3/schema.py

@@ -153,4 +153,4 @@ def _alter_many_to_many(self, model, old_field, new_field, strict):
             self.quote_name(old_field.rel.through._meta.db_table),
         ))
         # Delete the old through table
-        self.delete_model(old_field.rel.through, force=True)
+        self.delete_model(old_field.rel.through)

tests/modeltests/schema/tests.py

@@ -205,12 +205,28 @@ def test_alter(self):
             Author._meta.get_field_by_name("name")[0],
             new_field,
             strict=True,
-          )
+        )
         editor.commit()
         # Ensure the field is right afterwards
         columns = self.column_classes(Author)
         self.assertEqual(columns['name'][0], "TextField")
         self.assertEqual(columns['name'][1][6], True)
+        # Change nullability again
+        new_field2 = TextField(null=False)
+        new_field2.set_attributes_from_name("name")
+        editor = connection.schema_editor()
+        editor.start()
+        editor.alter_field(
+            Author,
+            new_field,
+            new_field2,
+            strict=True,
+        )
+        editor.commit()
+        # Ensure the field is right afterwards
+        columns = self.column_classes(Author)
+        self.assertEqual(columns['name'][0], "TextField")
+        self.assertEqual(columns['name'][1][6], False)
 
     def test_rename(self):
         """