def initial_data(cls, product_quantities):

        initial = {}

        for product, quantity in product_quantities:

            if quantity > 0:

                initial[cls.CHOICE_FIELD] = product.id

                initial[cls.QUANTITY_FIELD] = quantity

                break

        return initial

    def product_quantities(self):

        our_choice = self.cleaned_data[self.CHOICE_FIELD]

        our_quantity = self.cleaned_data[self.QUANTITY_FIELD]

        choices = self.fields[self.CHOICE_FIELD].choices

        for choice_value, choice_display in choices:

            if choice_value == 0:

                continue

            yield (

                choice_value,

                our_quantity if our_choice == choice_value else 0,

            )

    def add_product_error(self, product, error):

        if self.CHOICE_FIELD not in self.cleaned_data:

            return

        if product.id == self.cleaned_data[self.CHOICE_FIELD]:

            self.add_error(self.CHOICE_FIELD, error)

            self.add_error(self.QUANTITY_FIELD, error)

class _ItemQuantityProductsFormSet(_HasProductsFields, forms.BaseFormSet):

    @classmethod

    def set_fields(cls, category, products):

        raise ValueError("set_fields must be called on the underlying Form")

    @classmethod

    def initial_data(cls, product_quantities):

        ''' Prepares initial data for an instance of this form.

        product_quantities is a sequence of (product,quantity) tuples '''

        f = [

            {

                _ItemQuantityProductsForm.CHOICE_FIELD: product.id,

                _ItemQuantityProductsForm.QUANTITY_FIELD: quantity,

            }

            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")

    line_items = line_items.order_by(

        # sqlite requires an order_by for .values() to work

        "-price", "description",

    ).values(

        "price", "description",

    ).annotate(

        total_quantity=Sum("quantity"),

    )

    print line_items

    headings = ["Description", "Quantity", "Price", "Total"]

    data = []

    total_income = 0

    for line in line_items:

        cost = line["total_quantity"] * line["price"]

        data.append([

            line["description"], line["total_quantity"],

            line["price"], cost,

        ])

        total_income += cost

    data.append([

        "(TOTAL)", "--", "--", total_income,

    ])

    return Report("Paid items", headings, data)

@report_view("Product status", form_type=forms.ProductAndCategoryForm)

def product_status(request, form):

    ''' Summarises the inventory status of the given items, grouping by

    invoice status. '''

    products = form.cleaned_data["product"]

    categories = form.cleaned_data["category"]

    items = commerce.ProductItem.objects.filter(

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

    ).select_related("cart", "product")

    items = items.annotate(

        is_reserved=Case(

            When(cart__in=commerce.Cart.reserved_carts(), then=Value(1)),

            default=Value(0),

            output_field=models.BooleanField(),