models.py

from decimal import Decimal

from django.db import models
from django.utils.translation import ugettext_lazy as _


VAT_CHOICES = (
    (Decimal("0.00"), '0%'),
    (Decimal("2.10"), '2.1%'),
    (Decimal("5.50"), '5.5%'),
    (Decimal("8.50"), '8.5%'),
    (Decimal("10.00"), '10%'),
    (Decimal("20.00"), '20%'),
)

class Service(models.Model):
    name = models.CharField(max_length=255)
    buying_vat = models.DecimalField(_("buying VAT"),
        max_digits=8,
        decimal_places=2,
        choices=VAT_CHOICES,
    selling_vat = models.DecimalField(_("selling VAT"),
        max_digits=8,
        decimal_places=2,
        choices=VAT_CHOICES,


class Option(models.Model):
    name = models.CharField(max_length=255)
    services = models.ManyToManyField(Service, through="OptionService")


class OptionService(models.Model):
    option = models.ForeignKey(Option)
    service = models.ForeignKey(Service)
    price = models.DecimalField(_("Selling price with tax"),
        max_digits=8,
        decimal_places=2)