[Django] #24748: GROUP BY clause incorrect with foreign key to self in MySQL

[Django]

#24748: GROUP BY clause incorrect with foreign key to self in MySQL
----------------------------------------------+----------------------------
     Reporter:  sparkyb                       |      Owner:  nobody
         Type:  Bug                           |     Status:  new
    Component:  Database layer (models, ORM)  |    Version:  1.8
     Severity:  Normal                        |   Keywords:  annotate mysql
 Triage Stage:  Unreviewed                    |  Has patch:  0
Easy pickings:  0                             |      UI/UX:  0
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 A query I had that previously worked in Django 1.7 is quite broken in 1.8.
 When annotating a query of a model that has a a foreign key to itself, the
 wrong field is grouped by. This only occurs in MySQL due a bug to the
 group by primary key optimization.

 Using this example model:
 {{{
 class Item(Model):
     name = CharField(max_length=50)
     parent =
 ForeignKey('self',null=True,blank=True,related_name='children')
 }}}

 I used to be able to get a list of items with their number of children
 like this:
 {{{Item.objects.all().annotate(num_children=Count('children'))}}}

 But in Django 1.8, this groups by parent_id instead of by id. This is
 because both are selected, and the output field for both are the same, the
 primary key of Item.

--
Ticket URL: <https://code.djangoproject.com/ticket/24748>
Django <https://code.djangoproject.com/>
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