Django: Group model & count by date

model_stat_helper.py

"""
Generic method (disclaimer: I did not consider any security concerns) to get stats
on a model in Django.

Created from http://stackoverflow.com/questions/8746014/django-group-sales-by-month
"""
from django.db import connection
from django.db.models import Sum, Count

def truncate_date_model(model, period_length, date_field, criteria=None, pk='pk'):
    """
    model is the model class (Model, not instance nor model name as string)
    period_length is a string 'month', 'week', 'year' or 'quarter' for instance
    date_field is the string of the model's field to be truncated upon 'datetime', 'created'
    criteria is another optional field name, which may differentiate several series
    """
    truncate_date = connection.ops.date_trunc_sql(period_length, date_field)
    qs = model.objects.extra({period_length: truncate_date})
    if criteria:
        return qs.values(period_length, criteria).annotate(Count(criteria)).order_by(period_length)
    return qs.values(period_length).annotate(Count(pk)).order_by(period_length)