Add prefetch_related() cache invalidation for create() in reverse many-to-one managers.

[Rob Percival]

​The documentation says:

if you call the database-altering methods add(), remove(), clear() or set(), on related managers, any prefetched cache for the relation will be cleared.

This is accurate. However, there are other database-altering methods, such as create(), for which the cache ought to be cleared but isn't. This results in this confusing state of affairs:

x = MyModel.objects.prefetch_related("related_objects").get()
assert len(x.related_objects.all()) == 0
x.related_objects.create()
assert len(x.related_objects.all()) == 1  # Assertion fails because the prefetch cache isn't cleared by create()

Using add() rather than create() would cause the above code to work as expected. If there's a good reason for create() not clearing the cache, could that be documented please? Otherwise, could it clear the cache? From a search of Stack Overflow, this is definitely a source of confusion for some.

[David Sanders]

The original ticket #26706, ​ML thread & ​PR didn't seem to make any mention of create()

I'd agree this seems inconsistent but keen to see what felix & charettes (charettes reviewed the PR) think 🤔

[Mariusz Felisiak]

Thanks for the report, it seems that it's only missing for create() in reverse many-to-one managers. It works for many-to-many managers (we could add an extra test method for this, see ​PR) and GenericRelation (see #29612). See a regression test:

tests/many_to_one/tests.py

@@ -799 +799 @@
         # refs #21563
         self.assertFalse(hasattr(Article(), "reporter"))
 
+    def test_create_after_prefetch(self):
+        c = City.objects.create(name="Musical City")
+        d1 = District.objects.create(name="Ladida", city=c)
+        city = City.objects.prefetch_related("districts").get(id=c.id)
+        self.assertSequenceEqual(city.districts.all(), [d1])
+        d2 = city.districts.create(name="Goa")
+        self.assertSequenceEqual(city.districts.all(), [d1, d2])
+
     def test_clear_after_prefetch(self):
         c = City.objects.create(name="Musical City")
         d = District.objects.create(name="Ladida", city=c)

Rob, would you like to prepare a patch?

[GitHub <noreply@…>]

In 40a2c811:

Refs #26706, Refs #34633 -- Added test for prefetch_related() cache invalidation in ManyRelatedManager.create().

[Rob Percival]

Sure, I can look at creating a patch for this.

[Claude Paroz]

Please do not close a ticket before a patch is merged into the main branch.

[Aman Pandey]

PR up ​https://github.com/django/django/pull/17533

[Mariusz Felisiak <felisiak.mariusz@…>]

In 0fcd72b:

Fixed #34633 -- Made create() method of reverse many-to-one managers clear prefetch_related() cache.