Multiple calls to annotate returns only latest annotation when calling select_subclasses. #312

Problem

When making chained calls to annotate() on a single queryset, only the last annotation applied is returned after calling select_subclasses. If you don't call select_subclasses, you do receive all annotations.

Environment

• Django Model Utils version: 3.1.1
• Django version: 2.0
• Python version: 3.5.2
• Other libraries used, if any: n/a

Code examples

In the sample below, the only annotation that is available in the evaluated queryset is correct_count.

import django
django.setup()  # noqa

from django.db.models import Case, When, ExpressionWrapper, FloatField, Sum, Q

from sampleapp import models

correct_count_calc = ExpressionWrapper(
    Sum(
        Case(
            When((Q(answer__correct=True) & Q(answer__quizresult__user__is_superuser=False)), then=1),
            default=0
        )
    ), FloatField()
)

answered_count_calc = ExpressionWrapper(
    Sum(
        Case(
            When(answer__quizresult__user__is_superuser=False, then=1.0),
            default=0
        )
    ), FloatField()
)

q = models.BaseQuestion.objects.annotate(answered_count=answered_count_calc) \
                               .annotate(correct_count=correct_count_calc).select_subclasses().first()
print(q.__dict__)

It seems the problem stems from the snippet below, which sets (rather than appending to) the qset._annotated value for each subsequent call to .annotate().

I have a simple fix for this issue here:

https://github.com/digismack/django-model-utils/commit/6a72cebfc02fd19e68c15343fa9647256ad1e0f9

I was unable to get the test suite to run. They all fail with AppRegistryNotReady: Apps aren't loaded yet. If someone would be willing to help me out a bit, I'd love to submit a pull request with tests and such.