Allow to use DATE_TRUNC('month', "date_field") in PostgreSQL for Django 1.8+. I'm using this for django-oscar reports per month/week/year.

date_trunc.py

from django.db.models import Func, Value


class DateTrunc(Func):
    """
    To support using DATE_TRUNC('text', "field") in SQL
    
    Example::
    
        order_totals = (orders
            .annotate(
                period=DateTrunc('month', 'date_placed'),
            )
            .values("period")  # Needs to be in between for a correct GROUP_BY
            .order_by('period')
            .annotate(
                order_count=Count('id'),
                shipping_excl_tax=Sum('shipping_excl_tax'),
                shipping_incl_tax=Sum('shipping_incl_tax'),
            ))
    """
    function = 'DATE_TRUNC'

    def __init__(self, trunc_type, field_expression, **extra):
        super(DateTrunc, self).__init__(Value(trunc_type), field_expression, **extra)