QuerySet.only doesn't work with QuerySet.difference

[Dmitry Dygalo]

models.py:

class Model(models.Model):
    description = models.CharField(max_length=255)

Actual behavior:

>>> to_remove = Model.objects.filter(description='TO_REMOVE')
>>> query_set = Model.objects.all().difference(to_remove).only('pk')
>>> print(query_set.query)
(SELECT "test_model"."id", "test_model"."description" FROM "test_model") EXCEPT (SELECT "test_model"."id", "test_model"."description" FROM "test_model" WHERE "test_model"."description" = TO_REMOVE)

Expected:

>>> to_remove = Model.objects.filter(description='TO_REMOVE')
>>> query_set = Model.objects.all().difference(to_remove).only('pk')
>>> print(query_set.query)
(SELECT "test_model"."id" FROM "test_model") EXCEPT (SELECT "test_model"."id", "test_model"."description" FROM "test_model" WHERE "test_model"."description" = TO_REMOVE)

The problem:
Even if I use .only('pk') there is still "description" column in SELECT in the query, but I expect only "id" field as was specified in only.

[Tim Graham]

I believe it's unsupported but doesn't raise an error yet. As ​per the documentation, "only LIMIT, OFFSET, and ORDER BY (i.e. slicing and order_by()) are allowed on the resulting QuerySet." See #27995 for a ticket to raise a helpful message about this.