models.py

# sample implementation for http://stackoverflow.com/questions/37851053/django-query-group-by-month#new-answer

from __future__ import unicode_literals

from django.db import models
from django.db.models import Func, F, Sum
from django.db.transaction import atomic, rollback
from django.utils.timezone import now

class Invoice(models.Model):
    datetime = models.DateTimeField()
    total = models.IntegerField(default=100)

class Month(Func):
    function = 'EXTRACT'
    template = '%(function)s(MONTH from %(expressions)s)'
    output_field = models.IntegerField()

class MonthSqlite(Func):
    function = 'STRFTIME'
    template = '%(function)s("%%m", %(expressions)s)'
    output_field = models.CharField()

def test():
    TESTDATA = [
        (7, 300),
        (7, 200),
        (3, 100),
        (4, 500),
        (1, 100),
    ]

    with atomic():
        for m, total in TESTDATA:
            Invoice(datetime=now().replace(day=1, month=m), total=total).save()

        summary = (Invoice.objects
                      .annotate(m=MonthSqlite('datetime'))
                      .values('m')
                      .annotate(total=Sum('total'))
                      .order_by())

        for result in summary:
            print result

        raise Exception('stop')