prefetch_related_objects() does not set the `to_attr` on all instances if the first instance in the list already has it set

[Dennis Kliban]

Our project processes instances in a stream. In some cases the instances are repeated. In these cases, we discovered that prefetch_related_objects() does not set the to_attr on all of the instances if the first instance in the list already has it set.

When Django determines that the very first instance in the list is_fetched[0], it does not call into the the prefetch_one_level()[1]. The attributed specified in the to_attr parameter is only set in the prefetch_one_level() method[2].

is_fetched is set by looking for the to_attr attribute on the instance[3].

[0] ​https://github.com/django/django/blob/stable/2.2.x/django/db/models/query.py#L1605-L1609
[1] ​https://github.com/django/django/blob/stable/2.2.x/django/db/models/query.py#L1624
[2] ​https://github.com/django/django/blob/stable/2.2.x/django/db/models/query.py#L1799
[3] ​https://github.com/django/django/blob/stable/2.2.x/django/db/models/query.py#L1708