Queryset bug when using Q()? -- doubles up the counts by doing an extra join

[Javier Buzzi]

I ran this on all the "recent" versions of python and i see the issue on all of them. At this point, im not 100% sure if it is a problem or not, i ran across this trying to show some counts in the Admin that had some search_fields that traversed the model backwards and forwards in order to match the thing properly.

This "bug" can be seen from 1.11-3.0. Haven't checked 3.1 because i cant find it in pip.

This is my mocked models of the issue:

class ModelA(models.Model):
    somecriteria = models.CharField(max_length=50)


class ModelC(models.Model):
    unimportant = models.CharField(max_length=50)


class ModelB(models.Model):
    somerelation = models.ForeignKey(ModelA, on_delete=models.CASCADE)
    m2m = models.ManyToManyField(
        ModelC,
        through='ModelBC'
    )


class ModelBC(models.Model):
    b = models.ForeignKey(ModelB, on_delete=models.CASCADE)
    c = models.ForeignKey(ModelC, on_delete=models.CASCADE)

The "bug" is shown when i do:

queryset = ModelB.objects.select_related('a').annotate(num_c=Count('m2m__id')).values('num_c')
queryset = queryset.filter(Q(modelbc__c__unimportant='1') | Q(somerelation__somecriteria='1'))

When i do:

queryset = ModelB.objects.select_related('a').annotate(num_c=Count('m2m__id')).values('num_c')
queryset = queryset.filter(modelbc__c__unimportant='1', somerelation__somecriteria='1')

it works as expected.

My testing arena can be found here: ​https://gist.github.com/kingbuzzman/9fe5470e31a421aa88b2a64e5447e147 see the comments, for the way to run it.