Django is creating duplicate queries for prefetch_related

[Dheerendra Rathor]

Hello,

Recently I was monitoring db queries created by Django and a particular block was behaving unusual.

Here is the code block:

        from django.db import connection, reset_queries

        reset_queries()

        group = get_object_or_404(

            Group.objects.select_related('course').prefetch_related(

                Prefetch(

                    'concepts',

                    queryset=Concept.objects.filter(is_published=True).prefetch_related(

                        'videos', 'pages',

                    ),

                ),

            ).only('course', 'playlist'),

            pk=pk,

        )

        s4 = connection.queries

        k4 = len(s4)

        reset_queries()

Now, the interesting part is that prefetch queries are made twice for videos and pages. Hereis the query dump https://paste.kde.org/psrp4orn6

It is making same queries twice with exactly same parameters. 

Can someone please explain it for me? In my view, it should not happen. I'm missing something here? 

[Tim Graham]

Maybe https://code.djangoproject.com/ticket/25546 fixes it? This will be in Django 1.10.

[Dheerendra Rathor]

Thanks Tim, from the ticket and Patch it looks like this was my issue. 

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