Fix ValueError when calling prefetch_related for tracked ForeignKey fields

[joeriddles]

Problem

Related issue: #433

Running prefetch_related on a column which has a tracker will be failed with this error:

ValueError: "prefetch_related_field_name" does not resolve to an item that supports prefetching - this is an invalid parameter to prefetch_related().

When you call prefetch_related on a ForeignKey field that is being tracked, the following error below was being raised.

___________________________________________________________________ ModelTrackedPrefetchRelatedFKTests.test_default ____________________________________________________________________

self = <tests.test_fields.test_field_tracker.ModelTrackedPrefetchRelatedFKTests testMethod=test_default>

    def test_default(self):
        self.tracker = self.instance.tracker
>       list(self.tracked_class.objects.prefetch_related("fk__fk"))

tests/test_fields/test_field_tracker.py:553: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
.venv/lib/python3.8/site-packages/django/db/models/query.py:376: in __len__
    self._fetch_all()
.venv/lib/python3.8/site-packages/django/db/models/query.py:1869: in _fetch_all
    self._prefetch_related_objects()
.venv/lib/python3.8/site-packages/django/db/models/query.py:1258: in _prefetch_related_objects
    prefetch_related_objects(self._result_cache, *self._prefetch_related_lookups)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

model_instances = [<ModelTrackedPrefetchRelatedFK: ModelTrackedPrefetchRelatedFK object (1)>], related_lookups = ('fk__fk',), done_queries = {}, auto_lookups = set()
followed_descriptors = set()

    def prefetch_related_objects(model_instances, *related_lookups):
        ...
    
                if level == len(through_attrs) - 1 and prefetcher is None:
                    # Last one, this *must* resolve to something that supports
                    # prefetching, otherwise there is no point adding it and the
                    # developer asking for it has made a mistake.
>                   raise ValueError(
                        "'%s' does not resolve to an item that supports "
                        "prefetching - this is an invalid parameter to "
                        "prefetch_related()." % lookup.prefetch_through
                    )
E                   ValueError: 'fk__fk' does not resolve to an item that supports prefetching - this is an invalid parameter to prefetch_related().

.venv/lib/python3.8/site-packages/django/db/models/query.py:2287: ValueError
=============================================================================== short test summary info ================================================================================
FAILED tests/test_fields/test_field_tracker.py::ModelTrackedPrefetchRelatedFKTests::test_default - ValueError: 'fk__fk' does not resolve to an item that supports prefetching - this ...
========================================================================== 1 failed, 333 deselected in 1.12s ===========================================================================

Solution

Explain the solution that has been implemented, and what has been changed.

Commandments

• Write PEP8 compliant code.
• Cover it with tests.
• Update CHANGES.rst file to describe the changes, and quote according issue with GH-<issue_number>.
• Pay attention to backward compatibility, or if it breaks it, explain why.
• Update documentation (if relevant).

[Mogost]

Actually looks good. @foarsitter Can we merge this?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (11de64b) 95.08% compared to head (55cd816) 95.10%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #555      +/-   ##
==========================================
+ Coverage   95.08%   95.10%   +0.01%     
==========================================
  Files           6        6              
  Lines         794      796       +2     
==========================================
+ Hits          755      757       +2     
  Misses         39       39              

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[joeriddles]

How can I help get this merged? @Mogost @foarsitter

[foarsitter]

You can join jazzband and merge it. I'm not familiar enough with this so I can't merge it myself.