full_result_count optimization is wrong in the presence of custom ListFilters

[gwahl@…]

ChangeList.get_results performs an optimization when counting the total number of results, disregarding list filters. It assumes that if there are no query parameters, the full count is the same as the filtered count. This is incorrect when a custom list filter does filtering when there are no query parameters.

In my case, the list filter filters the objects by default, and removes the filtering with a query parameter. The optimization incorrectly assumes that there is no filtering by default, so the total result count is wrong.

​https://github.com/django/django/blob/master/django/contrib/admin/views/main.py#L172, see 'Perform a slight optimization'.

[Nick Sandford]

PR here: ​https://github.com/django/django/pull/5576.

[Tim Graham <timograham@…>]

In 5fa7b59:

Fixed #22810 -- Corrected admin changelist count for list filters that filter by default.