Use @cached_property for FieldCacheMixin cache key

[Adam Johnson]

FieldCacheMixin is used by related fields to track their cached values. Its existing design means calling get_cache_name() for each operation, even though that value doesn’t change. Changing get_cache_name() into a cached property can thus save a bunch of small but useless function calls working with related fields.

I profiled this change by using cProfile on a selection of tests using related fields:

$ python -m cProfile -s cumtime -o profile runtests.py --parallel 1 foreign_object *relat*
...
Found 399 test(s).
...

Before, there were 12,520 function calls:

$ python -m pstats profile <<< 'sort cumtime
stats 10000' | rg get_cache_name
    11193    0.001    0.000    0.001    0.000 /Users/chainz/Documents/Projects/django/django/db/models/fields/related.py:512(get_cache_name)
      712    0.000    0.000    0.000    0.000 /Users/chainz/Documents/Projects/django/django/db/models/fields/reverse_related.py:251(get_cache_name)
      615    0.000    0.000    0.000    0.000 /Users/chainz/Documents/Projects/django/django/contrib/contenttypes/fields.py:143(get_cache_name)

After, there are just 227 calls (should be one per related field):

$ python -m pstats profile <<< 'sort cumtime
stats 10000' | rg cache_name
      172    0.000    0.000    0.000    0.000 /Users/chainz/Documents/Projects/django/django/db/models/fields/related.py:512(cache_name)
       34    0.000    0.000    0.000    0.000 /Users/chainz/Documents/Projects/django/django/db/models/fields/reverse_related.py:251(cache_name)
       21    0.000    0.000    0.000    0.000 /Users/chainz/Documents/Projects/django/django/contrib/contenttypes/fields.py:143(cache_name)

The time saving is minimal here. It may be notable when working with a lot of model instances.

[Simon Charette]

It's a change that isn't too invasive so I think it's worth doing. I think we should add a deprecation shim for get_cache_name though.

[Simon Charette]

Patch is LGTM

[Natalia Bidart]

I agree the branch looks good but tests are missing, setting flags accordingly.