for product, quantity in product_quantities

            if quantity > 0

        ]

        return f

    def product_quantities(self):

        ''' Yields a sequence of (product, quantity) tuples from the

        cleaned form data. '''

        products = set()

        # Track everything so that we can yield some zeroes

        all_products = set()

        for form in self:

            if form.empty_permitted and not form.cleaned_data:

                # This is the magical empty form at the end of the list.

                continue

            for product, quantity in form.product_quantities():

                all_products.add(product)

                if quantity == 0:

                    continue

                if product in products:

                    form.add_error(

                        _ItemQuantityProductsForm.CHOICE_FIELD,

                        "You may only choose each product type once.",

                    )

                    form.add_error(

                        _ItemQuantityProductsForm.QUANTITY_FIELD,

                        "You may only choose each product type once.",

                    )

                products.add(product)

                yield product, quantity

        for product in (all_products - products):

            yield product, 0

    def add_product_error(self, product, error):

        for form in self.forms:

            form.add_product_error(product, error)

class VoucherForm(forms.Form):

    voucher = forms.CharField(

        label="Voucher code",

        help_text="If you have a voucher code, enter it here",

        required=False,

    )

from django.contrib.auth.decorators import user_passes_test

from django.core.urlresolvers import reverse

from django.db import models

from django.db.models import F, Q

from django.db.models import Sum

from django.db.models import Case, When, Value

from django.shortcuts import render

from registrasion.models import commerce

from registrasion import views

from reports import get_all_reports

from reports import Report

from reports import report_view

@user_passes_test(views._staff_only)

def reports_list(request):

    ''' Lists all of the reports currently available. '''

    reports = []

    for report in get_all_reports():

        reports.append({

            "name": report.__name__,

            "url": reverse(report),

            "description": report.__doc__,

        })

    reports.sort(key=lambda report: report["name"])

    ctx = {

        "reports": reports,

    }

    return render(request, "registrasion/reports_list.html", ctx)

# Report functions

@report_view("Paid items", form_type=forms.ProductAndCategoryForm)

def items_sold(request, form):

    ''' Summarises the items sold and discounts granted for a given set of

    products, or products from categories. '''

    data = None

    headings = None

    products = form.cleaned_data["product"]

    categories = form.cleaned_data["category"]

    line_items = commerce.LineItem.objects.filter(

        Q(product__in=products) | Q(product__category__in=categories),

        invoice__status=commerce.Invoice.STATUS_PAID,

    ).select_related("invoice")