id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
26565	Allow Prefetch query to use .values()	Maxime Lorant	nobody	"I came across a use case that does not seems to be possible using the Django ORM on Django 1.9. 

Let's says I want to get all objects from a `Order` model and prefetch for each order all pay lines, grouped per code, to get a list similar to: `[{'code': 'A', 'number': 3, 'amount': 1000}, {'code': 'B', 'number': 1, 'amount': 5000}]` if the order has four PayRollLine associated, with 3 where code=A and 1 where code=B.

{{{
# Query that will be used inside the `Prefetch` object later
# Let's assume quantity and unit_amount can't be null in the query 
# to avoid extra Coalesce...
prefetch_qs = (
    PayrollLine.objects.values('code').annotate(
        number=ExpressionWrapper(
            Sum(F('quantity')), output_field=DecimalField()
        ),
        amount=ExpressionWrapper(
            Sum(F('unit_amount') * F('quantity')), output_field=DecimalField()
        ),
    )
)
print(
    Order.objects.all()
    .prefetch_related(
        Prefetch('pay_lines', queryset=prefetch_qs, to_attr='pay_lines_grouped')
    )
)
}}}

The first query works fine alone, outside the prefetch object. But, when executing the second query, Django raises an exception:

{{{
    Traceback (most recent call last):
      [...]
      File ""/data/.virtualenvs/ORFEO/local/lib/python2.7/site-packages/django/db/models/query.py"", line 234, in __repr__
        data = list(self[:REPR_OUTPUT_SIZE + 1])
      File ""/data/.virtualenvs/ORFEO/local/lib/python2.7/site-packages/django/db/models/query.py"", line 258, in __iter__
        self._fetch_all()
      File ""/data/.virtualenvs/ORFEO/local/lib/python2.7/site-packages/django/db/models/query.py"", line 1076, in _fetch_all
        self._prefetch_related_objects()
      File ""/data/.virtualenvs/ORFEO/local/lib/python2.7/site-packages/django/db/models/query.py"", line 656, in _prefetch_related_objects
        prefetch_related_objects(self._result_cache, self._prefetch_related_lookups)
      File ""/data/.virtualenvs/ORFEO/local/lib/python2.7/site-packages/django/db/models/query.py"", line 1457, in prefetch_related_objects
        obj_list, additional_lookups = prefetch_one_level(obj_list, prefetcher, lookup, level)
      File ""/data/.virtualenvs/ORFEO/local/lib/python2.7/site-packages/django/db/models/query.py"", line 1556, in prefetch_one_level
        prefetcher.get_prefetch_queryset(instances, lookup.get_current_queryset(level)))
      File ""/data/.virtualenvs/ORFEO/local/lib/python2.7/site-packages/django/db/models/fields/related_descriptors.py"", line 544, in get_prefetch_queryset
        instance = instances_dict[rel_obj_attr(rel_obj)]
      File ""/data/.virtualenvs/ORFEO/local/lib/python2.7/site-packages/django/db/models/fields/related.py"", line 597, in get_local_related_value
        return self.get_instance_value_for_fields(instance, self.local_related_fields)
      File ""/data/.virtualenvs/ORFEO/local/lib/python2.7/site-packages/django/db/models/fields/related.py"", line 605, in get_instance_value_for_fields
        opts = instance._meta
    AttributeError: 'dict' object has no attribute '_meta'
}}}

Looks like Django can't work with dict in prefetch_related... On IRC, someone point me to [this PR](https://github.com/django/django/pull/6478) that could resolve my problem but I think it should be verified with unit tests."	New feature	new	Database layer (models, ORM)	dev	Normal		prefetch, values	Dan LaManna Micah Lyle Thomas Riccardi Ülgen Sarıkavak	Accepted	0	0	0	0	0	0