Many to many

src/computed_field/fields.py

@@ -3,6 +3,7 @@
 import django
 from django.db import models
 from django.db.models.expressions import Col
+from django.db.models.aggregates import Aggregate
 
 
 class ComputedField(models.Field):
@@ -83,6 +84,11 @@ def resolve_f(expression, query):
                     return resolve_f(field.expression, query)
                 return Col(join_list[-1], field)
 
+            if isinstance(expression, Aggregate):
+                if query.group_by:
+                    query.group_by += (self.model._meta.pk.name,)
+                else:
+                    query.group_by = (self.model._meta.pk.name,)
             return expression
 
         # I'd love some way to get the query object without having to peek up the stack...

tests/models.py

@@ -18,6 +18,7 @@ class Person(models.Model):
     user = models.ForeignKey(User, null=True, blank=True, on_delete=models.CASCADE)
     first_name = models.TextField()
     last_name = models.TextField()
+    follows = models.ManyToManyField('tests.Person', related_name='followed_by')
 
     name = ComputedField(Concat(
         models.F('first_name'), Value(' '), models.F('last_name'),
@@ -29,6 +30,8 @@ class Person(models.Model):
     username = ComputedField(models.F('user__username'))
     group = ComputedField(models.F('user__group__name'))
 
+    follower_names = ComputedField(models.F('follows__name'))
+
     class Meta:
         indexes = [
             # models.Index(fields=['lower_name']),
@@ -37,3 +40,10 @@ class Meta:
 
 class Address(models.Model):
     person = models.OneToOneField(Person, related_name='address', primary_key=True, on_delete=models.CASCADE)
+
+
+class Location(models.Model):
+    name = models.TextField()
+    people = models.ManyToManyField(Person, related_name='locations')
+
+    person_count = ComputedField(models.Count('people'))

tests/tests/test_lookup_expression.py

@@ -1,4 +1,4 @@
-from ..models import Person, User, Group
+from ..models import Person, User, Group, Location
 
 
 def test_empty_lookup():
@@ -32,3 +32,23 @@ def test_multiple_lookups():
     assert Person.objects.get().group == 'qux'
     assert Person.objects.filter(group='qux').exists()
     assert not Person.objects.filter(group=None).exists()
+
+
+def test_many_to_many():
+    alice = Person.objects.create(first_name='Alice', last_name='A')
+    bob = Person.objects.create(first_name='Bob', last_name='B')
+    carol = Person.objects.create(first_name='Carol', last_name='C')
+
+    one = Location.objects.create(name='One')
+    two = Location.objects.create(name='Two')
+    three = Location.objects.create(name='Three')
+
+    one.people.add(alice, bob)
+    two.people.add(carol, alice, bob)
+
+    locations = Location.objects.order_by('person_count')
+    assert False == str(locations.query)
+    assert len(locations) == 3
+    assert locations[0] == three
+    assert locations[1] == one
+    assert locations[2] == two

tox.ini

@@ -3,15 +3,12 @@ envlist=
   flake8,
   isort,
   bandit,
-  py{36,37}-django{20,21,dev}-{base,flake8,prospector},
+  py{36,37}-django{20,21,22,dev}-{base,flake8,prospector},
   py{27,36,37}-django11-{base,flake8,prospector}
 skip_missing_interpreters=true
 
-[travis]
-python=
-  2.7: py27
-  3.6: py36
-  3.7: py37
+[tox:travis]
+3.6 = py36, flake8
 
 [pytest]
 DJANGO_SETTINGS_MODULE = tests.settings
@@ -30,7 +27,9 @@ deps=
 deps=
   django11: Django>=1.11,<2.0
   django20: Django>=2.0,<2.1
-  django21: Django>=2.1
+  django21: Django>=2.1.11,<2.2
+  django22: Django>=2.2,<3.0
+  django30: Django>=3.0
   djangodev: https://github.com/django/django/archive/master.tar.gz
 
 [testenv]