Remove Error raising on annotation & distinct call

[Yovel Cohen]

if you try to call distinct on an annotated query, it sometimes works, sometimes get's ignore, and sometimes raises an error.
I had a query facing all three scenarios and solving the last one was the one that worked out.

the error being raised is:

NotImplementedError: annotate() + distinct(fields) is not implemented.

after looking at the source code for the exception at django.db.models.sql.compiler.SQLCompiler
row 594

if grouping:
        if distinct_fields:
           raise NotImplementedError('annotate() + distinct(fields) is not implemented.')
        order_by = order_by or self.connection.ops.force_no_ordering()
        result.append('GROUP BY %s' % ', '.join(grouping))
# rest of the as_sql() method

after just removing the distinct_fields condition:

if grouping:
        order_by = order_by or self.connection.ops.force_no_ordering()
        result.append('GROUP BY %s' % ', '.join(grouping))
# rest of the as_sql() method

it works, at least in the following ways I tried (annotations are just made up for sake of the example)

model_scores_latest_date_annotation = Max('model_scores__date')
latest_score_annotation = Case(When(model_scores__date=F('latest_date'), then='model_scores__score')
base_query_set = base_query_set = (Model.objects.
                  filter(**filters).
                  alias(latest_date=model_scores_latest_date_annotation).
                  values(ID).
                  annotate(latest_score=latest_score_annotation, latest_date=model_scores_latest_date_annotation)
                  )

all the following distinct calls worked:

query_set = base_query_set.order_by('latest_date').distinct('id', 'latest_date')

query_set = base_query_set.distinct('id')

query_set = base_query_set.distinct('id', 'latest_date')

which makes me think that the as_sql method on SQLCompiler can handle more cases easily and this just fell beneath the cracks.