When calling `select_related()` with an empty list of arguments [1], Django will
try to prefetch some data by doing some first level joints with the related
classes.
This can lead to obvious negative performance impact, but this also breaks some
workarounds for having inheritance for foreign keys [2], as those solutions rely
on lazy evaluation of the related object.
[1]: a4e6030904/django/db/models/query.py (L1051)
Only passing an explicit `None` to `select_related` will disable the magic.
[2]: https://github.com/jazzband/django-model-utils/issues/11
As examples, here are the generated SQL requests in
InheritanceManagerRelatedTests.test_get_method_with_select_subclasses_check_for_useless_join:
* without this fix, without adding `.select_related(None)`
```sql
SELECT
"tests_inheritancemanagertestparent"."id",
"tests_inheritancemanagertestparent"."non_related_field_using_descriptor",
"tests_inheritancemanagertestparent"."related_id",
"tests_inheritancemanagertestparent"."normal_field",
"tests_inheritancemanagertestparent"."related_self_id",
"tests_inheritancemanagertestchild4"."other_onetoone_id",
"tests_inheritancemanagertestchild4"."parent_ptr_id", T3."id",
T3."non_related_field_using_descriptor", T3."related_id", T3."normal_field",
T3."related_self_id"
FROM
"tests_inheritancemanagertestchild4"
INNER JOIN
"tests_inheritancemanagertestparent" ON
("tests_inheritancemanagertestchild4"."parent_ptr_id" = "tests_inheritancemanagertestparent"."id")
INNER JOIN
"tests_inheritancemanagertestparent" T3 ON
("tests_inheritancemanagertestchild4"."other_onetoone_id" = T3."id")
WHERE
"tests_inheritancemanagertestchild4"."parent_ptr_id" = 191
```
* with either the fix, or by adding `.select_related(None)` after `.select_subclasses()`
```sql
SELECT
"tests_inheritancemanagertestparent"."id",
"tests_inheritancemanagertestparent"."non_related_field_using_descriptor",
"tests_inheritancemanagertestparent"."related_id",
"tests_inheritancemanagertestparent"."normal_field",
"tests_inheritancemanagertestparent"."related_self_id",
"tests_inheritancemanagertestchild4"."other_onetoone_id",
"tests_inheritancemanagertestchild4"."parent_ptr_id"
FROM
"tests_inheritancemanagertestchild4"
INNER JOIN
"tests_inheritancemanagertestparent" ON
("tests_inheritancemanagertestchild4"."parent_ptr_id" = "tests_inheritancemanagertestparent"."id")
WHERE
"tests_inheritancemanagertestchild4"."parent_ptr_id" = 191
```
I noticed this discrepancy while attempting to determine compatibility of versions of django-model-utils and Django. I haven't tested it locally, because I'm getting `psycopg2.OperationalError: could not connect to server`.
* Overriding TimeStampedModel's save() method
to update modified field automatically when update_fields argument is used but modified was not given as a parameter.
* fixed a small error in logic
* added tests for new functionality
