[Django] #27159: Pickling query from queryset causes unintended queryset evaluation

[Django]

#27159: Pickling query from queryset causes unintended queryset evaluation
-------------------------------+--------------------
     Reporter:  jtiai          |      Owner:  nobody
         Type:  Uncategorized  |     Status:  new
    Component:  Uncategorized  |    Version:  1.10
     Severity:  Normal         |   Keywords:
 Triage Stage:  Unreviewed     |  Has patch:  0
Easy pickings:  0              |      UI/UX:  0
-------------------------------+--------------------
 According to documentation pickling queryset.query should be correct
 approach and it should only pickle only relevant information to recreate
 queryset.query again.

 Given models:
 {{{#!python
 from django.db import models

 # Create your models here.
 class ParentModel(models.Model):
     name = models.CharField(max_length=64)

 class ChildModel(models.Model):
     name = models.CharField(max_length=64)

     parent = models.ForeignKey('ParentModel', related_name='children')
 }}}

 Following querys, when pickled causes unwanted evaluation and creating
 quite large pickled result:
 {{{#!python
 for x in range(1,10000):
     ParentModel(name='Parent {}'.format(x), ).save()

 ChildModel(name='Child 1', parent=ParentModel.objects.all()[0]).save()

 parents_1 = ParentModel.objects.all().values_list('pk', flat=True)
 children_1 = ChildModel.objects.filter(parent__in=parents_1)

 pickled_stuff_1 = pickle.dumps(children_1.query)

 parents_2 = ParentModel.objects.all()
 children_2 = ChildModel.objects.filter(parent__in=parents_2)

 pickled_stuff_2 = pickle.dumps(children_2.query)

 # First len is about 74 kilobytes, second len is about 2 megabytes.
 print len(pickled_stuff_1), len(pickled_stuff_2)
 }}}

 When comparing sizes of pickled queries both are relatively big. When
 inspecting latter pickle it can be seen that it actually contains all
 instances from fully evaluated queryset. Same behavior exists in 1.8 and
 1.10 as well.

--
Ticket URL: <https://code.djangoproject.com/ticket/27159>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.

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