PostgreSQL's ArrayField "in" operator does not allow subqueries

[Michał Pasternak]

Hi,

I have a legitimate use-case, where I want to filter using "in" operator a PostgreSQL ArrayField (contrib.postgres.fields.array.ArrayField).

Current implementation does not allow that.

You will get a traceback similar to the one below:

Traceback (most recent call last):
  File "/Users/mpasternak/Programowanie/django/tests/postgres_tests/test_array.py", line 185, in test_in_subquery
    "field", flat=True)
  File "/Users/mpasternak/Programowanie/django/django/db/models/manager.py", line 82, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/Users/mpasternak/Programowanie/django/django/db/models/query.py", line 835, in filter
    return self._filter_or_exclude(False, *args, **kwargs)
  File "/Users/mpasternak/Programowanie/django/django/db/models/query.py", line 853, in _filter_or_exclude
    clone.query.add_q(Q(*args, **kwargs))
  File "/Users/mpasternak/Programowanie/django/django/db/models/sql/query.py", line 1252, in add_q
    clause, _ = self._add_q(q_object, self.used_aliases)
  File "/Users/mpasternak/Programowanie/django/django/db/models/sql/query.py", line 1276, in _add_q
    split_subq=split_subq,
  File "/Users/mpasternak/Programowanie/django/django/db/models/sql/query.py", line 1214, in build_filter
    condition = self.build_lookup(lookups, col, value)
  File "/Users/mpasternak/Programowanie/django/django/db/models/sql/query.py", line 1084, in build_lookup
    lookup = lookup_class(lhs, rhs)
  File "/Users/mpasternak/Programowanie/django/django/db/models/lookups.py", line 18, in __init__
    self.rhs = self.get_prep_lookup()
  File "/Users/mpasternak/Programowanie/django/django/contrib/postgres/fields/array.py", line 252, in get_prep_lookup
    for value in values:
TypeError: 'Query' object is not iterable

The fix seems trivial.

I implemented both the fix and the test for it in my branch here: ​https://github.com/mpasternak/django .

I am submitting a pull request on GitHub in a moment.

I am planning to back port this to 1.11.x .

Not sure if this is a Bug or a Feature, classifying as Bug for now.

This is my first contribution to Django Project that, I believe, is going to be merged. I have sent CLA as requested.