managers: avoid querying too much tables when not needed

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

```

CHANGES.rst

@@ -3,6 +3,7 @@ CHANGES
 
 4.0.1 (unreleased)
 ------------------
+- Update InheritanceQuerySetMixin to avoid querying too much tables
 - TimeStampedModel now automatically adds 'modified' field as an update_fields
   parameter even if it is forgotten while using save()
 - `FieldTracker` now marks fields as not changed after `refresh_from_db`

model_utils/managers.py

@@ -75,7 +75,10 @@ class InheritanceQuerySetMixin:
 
         # workaround https://code.djangoproject.com/ticket/16855
         previous_select_related = self.query.select_related
-        new_qs = self.select_related(*subclasses)
+        if subclasses:
+            new_qs = self.select_related(*subclasses)
+        else:
+            new_qs = self
         previous_is_dict = isinstance(previous_select_related, dict)
         new_is_dict = isinstance(new_qs.query.select_related, dict)
         if previous_is_dict and new_is_dict:

tests/test_managers/test_inheritance_manager.py

@@ -487,6 +487,14 @@ class InheritanceManagerRelatedTests(InheritanceManagerTests):
                 id=self.child1.id),
             self.child1)
 
+    def test_get_method_with_select_subclasses_check_for_useless_join(self):
+        child4 = InheritanceManagerTestChild4.objects.create(related=self.related, other_onetoone=self.child1)
+        self.assertEqual(
+            str(InheritanceManagerTestChild4.objects.select_subclasses().filter(
+                id=child4.id).query),
+            str(InheritanceManagerTestChild4.objects.select_subclasses().select_related(None).filter(
+                id=child4.id).query))
+
     def test_annotate_with_select_subclasses(self):
         qs = InheritanceManagerTestParent.objects.select_subclasses().annotate(
             models.Count('id'))