Re: .count() returning unexpected results

On Thu, Oct 20, 2011 at 12:40 PM, Tim Chase
<django.users@tim.thechases.com> wrote:
> On 10/19/11 20:35, Tim Chase wrote:
>
> Perhaps I should have changed the test to be more clear, instead of using
>
>>   results = m.Person.objects.filter(
>>       Q(name__iexact=alias) |
>>       Q(aliases__alias__iexact=alias)
>>       )
>>   self.assertEqual(1, results.count(),
>>       "Failed #%i on %r" % (i, alias),)
>
> use
>
>  self.assertEqual(
>     m.Person.objects.count(), #only 1
>     results.count(), # comes back as 6 for "William"
>     "Failed #%i on %r" % (i, alias),
>     )
>
> This makes it clearer that there's only the one person in the DB (no
> fixtures being loaded).  The underlying SQL brings back the number of rows
> caused by the join of Person with Alias, but it seems a bug that
> .objects.filter().count() should ever return more than .objects.count()
>
> -tkc
>

It might seem that way, but it is definitely not a bug.

count() counts the number of rows found by the query, and your query
can find the same row multiple times, as you are joining across a M2M
relationship.

If you want distinct results from the query, make sure that you tell
django so by using distinct().

Eg:

>>> Type.objects.filter(department__backend_type__name='Other')
[<Type: Manufacturer>, <Type: Manufacturer>, <Type: Manufacturer>,
<Type: Manufacturer>, <Type: Manufacturer>]
>>> Type.objects.filter(department__backend_type__name='Other').count()
5
>>> Type.objects.filter(department__backend_type__name='Other').distinct()
[<Type: Manufacturer>]
>>> Type.objects.filter(department__backend_type__name='Other').distinct().count()
1

Cheers

Tom

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