uncloud/uncloud_pay/models.py

from django.db import models
from django.db.models import Q
from django.contrib.auth import get_user_model
from django.utils.translation import gettext_lazy as _
from django.core.validators import MinValueValidator
from django.utils import timezone
from django.core.exceptions import ObjectDoesNotExist, ValidationError

from django.contrib.contenttypes.fields import GenericForeignKey
from django.contrib.contenttypes.models import ContentType

import logging
from functools import reduce
import itertools
from math import ceil
import datetime
from calendar import monthrange
from decimal import Decimal

import uncloud_pay.stripe
from uncloud_pay.helpers import beginning_of_month, end_of_month
from uncloud_pay import AMOUNT_DECIMALS, AMOUNT_MAX_DIGITS, COUNTRIES
from uncloud.models import UncloudModel, UncloudStatus

from decimal import Decimal
import decimal

# Used to generate bill due dates.
BILL_PAYMENT_DELAY=datetime.timedelta(days=10)

# Initialize logger.
logger = logging.getLogger(__name__)

def start_of_month(a_day):
    """ Returns first of the month of a given datetime object"""
    return a_day.replace(day=1,hour=0,minute=0,second=0, microsecond=0)

def end_of_month(a_day):
    """ Returns first of the month of a given datetime object"""

    _, last_day = monthrange(a_day.year, a_day.month)
    return a_day.replace(day=last_day,hour=23,minute=59,second=59, microsecond=0)

def start_of_this_month():
    """ Returns first of this month"""
    a_day = timezone.now()
    return a_day.replace(day=1,hour=0,minute=0,second=0, microsecond=0)

def end_of_this_month():
    """ Returns first of this month"""
    a_day = timezone.now()

    _, last_day = monthrange(a_day.year, a_day.month)
    return a_day.replace(day=last_day,hour=23,minute=59,second=59, microsecond=0)

def end_before(a_date):
    """ Return suitable datetimefield for ending just before a_date """
    return a_date - datetime.timedelta(seconds=1)

def start_after(a_date):
    """ Return suitable datetimefield for starting just after a_date """
    return a_date + datetime.timedelta(seconds=1)

def default_payment_delay():
    return timezone.now() + BILL_PAYMENT_DELAY


class Currency(models.TextChoices):
    """
    We don't support months are years, because they vary in length.
    This is not only complicated, but also unfair to the user, as the user pays the same
    amount for different durations.
    """
    CHF   = 'CHF', _('Swiss Franc')
    EUR   = 'EUR', _('Euro')
    USD   = 'USD', _('US Dollar')

class CountryField(models.CharField):
    def __init__(self, *args, **kwargs):
        kwargs.setdefault('choices', COUNTRIES)
        kwargs.setdefault('default', 'CH')
        kwargs.setdefault('max_length', 2)

        super().__init__(*args, **kwargs)

    def get_internal_type(self):
        return "CharField"

def get_balance_for_user(user):
    bills = reduce(
            lambda acc, entry: acc + entry.total,
            Bill.objects.filter(owner=user),
            0)
    payments = reduce(
            lambda acc, entry: acc + entry.amount,
            Payment.objects.filter(owner=user),
            0)
    return payments - bills

class StripeCustomer(models.Model):
    owner = models.OneToOneField( get_user_model(),
            primary_key=True,
            on_delete=models.CASCADE)
    stripe_id = models.CharField(max_length=32)

###
# Payments and Payment Methods.

class Payment(models.Model):
    owner = models.ForeignKey(get_user_model(),
            on_delete=models.CASCADE)

    amount = models.DecimalField(
            default=0.0,
            max_digits=AMOUNT_MAX_DIGITS,
            decimal_places=AMOUNT_DECIMALS,
            validators=[MinValueValidator(0)])

    source = models.CharField(max_length=256,
                              choices = (
                                  ('wire', 'Wire Transfer'),
                                  ('stripe', 'Stripe'),
                                  ('voucher', 'Voucher'),
                                  ('referral', 'Referral'),
                                  ('unknown', 'Unknown')
                              ),
                              default='unknown')
    timestamp = models.DateTimeField(editable=False, auto_now_add=True)

    # We override save() in order to active products awaiting payment.
    def save(self, *args, **kwargs):
        # _state.adding is switched to false after super(...) call.
        being_created = self._state.adding

        unpaid_bills_before_payment = Bill.get_unpaid_for(self.owner)
        super(Payment, self).save(*args, **kwargs) # Save payment in DB.
        unpaid_bills_after_payment = Bill.get_unpaid_for(self.owner)

        newly_paid_bills = list(
                set(unpaid_bills_before_payment) - set(unpaid_bills_after_payment))
        for bill in newly_paid_bills:
            bill.activate_products()


class PaymentMethod(models.Model):
    owner = models.ForeignKey(get_user_model(),
            on_delete=models.CASCADE,
            editable=False)
    source = models.CharField(max_length=256,
            choices = (
                ('stripe', 'Stripe'),
                ('unknown', 'Unknown'),
                ),
            default='stripe')
    description = models.TextField()
    primary = models.BooleanField(default=False, editable=False)

    # Only used for "Stripe" source
    stripe_payment_method_id = models.CharField(max_length=32, blank=True, null=True)
    stripe_setup_intent_id = models.CharField(max_length=32, blank=True, null=True)

    @property
    def stripe_card_last4(self):
        if self.source == 'stripe' and self.active:
            payment_method = uncloud_pay.stripe.get_payment_method(
                    self.stripe_payment_method_id)
            return payment_method.card.last4
        else:
            return None

    @property
    def active(self):
        if self.source == 'stripe' and self.stripe_payment_method_id != None:
            return True
        else:
            return False

    def charge(self, amount):
        if not self.active:
            raise Exception('This payment method is inactive.')

        if amount < 0: # Make sure we don't charge negative amount by errors...
            raise Exception('Cannot charge negative amount.')

        if self.source == 'stripe':
            stripe_customer = StripeCustomer.objects.get(owner=self.owner).stripe_id
            stripe_payment = uncloud_pay.stripe.charge_customer(
                    amount, stripe_customer, self.stripe_payment_method_id)
            if 'paid' in stripe_payment and stripe_payment['paid'] == False:
                raise Exception(stripe_payment['error'])
            else:
                payment = Payment.objects.create(
                        owner=self.owner, source=self.source, amount=amount)

                return payment
        else:
            raise Exception('This payment method is unsupported/cannot be charged.')

    def set_as_primary_for(self, user):
        methods = PaymentMethod.objects.filter(owner=user, primary=True)
        for method in methods:
            print(method)
            method.primary = False
            method.save()

        self.primary = True
        self.save()

    def get_primary_for(user):
        methods = PaymentMethod.objects.filter(owner=user)
        for method in methods:
            # Do we want to do something with non-primary method?
            if method.active and method.primary:
                return method

        return None

    class Meta:
        # TODO: limit to one primary method per user.
        # unique_together is no good since it won't allow more than one
        # non-primary method.
        pass

# See https://docs.djangoproject.com/en/dev/ref/models/fields/#field-choices-enum-types
class RecurringPeriodChoices(models.IntegerChoices):
    """
    This is an old class and being superseeded by the database model below
    """
    PER_365D   = 365*24*3600, _('Per 365 days')
    PER_30D    = 30*24*3600, _('Per 30 days')
    PER_WEEK   = 7*24*3600, _('Per Week')
    PER_DAY    = 24*3600, _('Per Day')
    PER_HOUR   = 3600, _('Per Hour')
    PER_MINUTE = 60, _('Per Minute')
    PER_SECOND = 1, _('Per Second')
    ONE_TIME   = 0, _('Onetime')

# RecurringPeriods
class RecurringPeriod(models.Model):
    """
    Available recurring periods.
    By default seeded from RecurringPeriodChoices
    """

    name = models.CharField(max_length=100, unique=True)
    duration_seconds = models.IntegerField(unique=True)

    @classmethod
    def populate_db_defaults(cls):
        for (seconds, name) in RecurringPeriodChoices.choices:
            obj, created = cls.objects.get_or_create(name=name,
                                                     defaults={ 'duration_seconds': seconds })


    def __str__(self):
        return f"{self.name} ({self.duration_seconds})"


###
# Bills.

class BillingAddress(models.Model):
    owner = models.ForeignKey(get_user_model(), on_delete=models.CASCADE)

    organization = models.CharField(max_length=100, blank=True, null=True)
    name = models.CharField(max_length=100)
    street = models.CharField(max_length=100)
    city = models.CharField(max_length=50)
    postal_code = models.CharField(max_length=50)
    country = CountryField(blank=True)
    vat_number = models.CharField(max_length=100, default="", blank=True)
    active = models.BooleanField(default=False)

    class Meta:
        constraints = [
            models.UniqueConstraint(fields=['owner'],
                                    condition=Q(active=True),
                                    name='one_active_billing_address_per_user')
        ]

    @staticmethod
    def get_address_for(user):
        return BillingAddress.objects.get(owner=user, active=True)

    def __str__(self):
        return "{} - {}, {}, {} {}, {}".format(
            self.owner,
            self.name, self.street, self.postal_code, self.city,
            self.country)

###
# VAT

class VATRate(models.Model):
    starting_date = models.DateField(blank=True, null=True)
    ending_date = models.DateField(blank=True, null=True)
    territory_codes = models.TextField(blank=True, default='')
    currency_code = models.CharField(max_length=10)
    rate = models.FloatField()
    rate_type = models.TextField(blank=True, default='')
    description = models.TextField(blank=True, default='')

    @staticmethod
    def get_for_country(country_code):
        vat_rate = None
        try:
            vat_rate = VATRate.objects.get(
                territory_codes=country_code, start_date__isnull=False, stop_date=None
            )
            return vat_rate.rate
        except VATRate.DoesNotExist as dne:
            logger.debug(str(dne))
            logger.debug("Did not find VAT rate for %s, returning 0" % country_code)
            return 0

###
# Products

class Product(UncloudModel):
    """
    A product is something a user can order. To record the pricing, we
    create order that define a state in time.

    A product can have *one* one_time_order and/or *one*
    recurring_order.

    If either of them needs to be updated, a new order of the same
    type will be created and links to the previous order.

    """

    name = models.CharField(max_length=256)
    description = models.CharField(max_length=1024)

    config = models.JSONField()

#    default_recurring_period = models.IntegerField(choices=RecurringPeriod.choices, default=RecurringPeriod.PER_30D)
    default_recurring_period = models.ForeignKey(RecurringPeriod, on_delete=models.CASCADE, editable=True)


    currency = models.CharField(max_length=32, choices=Currency.choices, default=Currency.CHF)

    @property
    def recurring_orders(self):
        return self.orders.order_by('id').exclude(recurring_period=RecurringPeriod.ONE_TIME)

    @property
    def last_recurring_order(self):
        return self.recurring_orders.last()

    @property
    def one_time_orders(self):
        return self.orders.order_by('id').filter(recurring_period=RecurringPeriod.ONE_TIME)

    @property
    def last_one_time_order(self):
        return self.one_time_orders.last()

    def create_order(self, when_to_start=None, recurring_period=None):
        billing_address = BillingAddress.get_address_for(self.owner)

        if not billing_address:
            raise ValidationError("Cannot order without a billing address")

        if not when_to_start:
            when_to_start = timezone.now()

        if not recurring_period:
            recurring_period = self.default_recurring_period


        # Create one time order if we did not create one already
        if self.one_time_price > 0 and not self.last_one_time_order:
            one_time_order = Order.objects.create(owner=self.owner,
                                                  billing_address=billing_address,
                                                  starting_date=when_to_start,
                                                  price=self.one_time_price,
                                                  recurring_period=RecurringPeriod.ONE_TIME,
                                                  description=str(self))
            self.orders.add(one_time_order)
        else:
            one_time_order = None

        if recurring_period != RecurringPeriod.ONE_TIME:
            if one_time_order:
                recurring_order = Order.objects.create(owner=self.owner,
                                                       billing_address=billing_address,
                                                       starting_date=when_to_start,
                                                       price=self.recurring_price,
                                                       recurring_period=recurring_period,
                                                       depends_on=one_time_order,
                                                       description=str(self))
            else:
                recurring_order = Order.objects.create(owner=self.owner,
                                                       billing_address=billing_address,
                                                       starting_date=when_to_start,
                                                       price=self.recurring_price,
                                                       recurring_period=recurring_period,
                                                       description=str(self))
            self.orders.add(recurring_order)


    # FIXME: this could/should be part of Order (?)
    def create_or_update_recurring_order(self, when_to_start=None, recurring_period=None):
        if not self.recurring_price:
            return

        if not recurring_period:
            recurring_period = self.default_recurring_period

        if not when_to_start:
            when_to_start = timezone.now()

        if self.last_recurring_order:
            if self.recurring_price < self.last_recurring_order.price:

                if when_to_start < self.last_recurring_order.next_cancel_or_downgrade_date:
                    when_to_start = start_after(self.last_recurring_order.next_cancel_or_downgrade_date)

            when_to_end = end_before(when_to_start)

            new_order = Order.objects.create(owner=self.owner,
                                             billing_address=self.last_recurring_order.billing_address,
                                             starting_date=when_to_start,
                                             price=self.recurring_price,
                                             recurring_period=recurring_period,
                                             description=str(self),
                                             replaces=self.last_recurring_order)

            self.last_recurring_order.replace_with(new_order)
            self.orders.add(new_order)
        else:
            self.create_order(when_to_start, recurring_period)


    @property
    def recurring_price(self):
        """ implement correct values in the child class """
        return 0

    @property
    def one_time_price(self):
        """ implement correct values in the child class """
        return 0


    @property
    def is_recurring(self):
        return self.recurring_price > 0

    @property
    def billing_address(self):
        return self.order.billing_address

    @staticmethod
    def allowed_recurring_periods():
        return RecurringPeriod.choices

    # class Meta:
    #     abstract = True

    def discounted_price_by_period(self, requested_period):
        """
        Each product has a standard recurring period for which
        we define a pricing. I.e. VPN is usually year, VM is usually monthly.

        The user can opt-in to use a different period, which influences the price:
        The longer a user commits, the higher the discount.

        Products can also be limited in the available periods. For instance
        a VPN only makes sense to be bought for at least one day.

        Rules are as follows:

        given a standard recurring period of ..., changing to ... modifies price ...


        # One month for free if buying / year, compared to a month: about 8.33% discount
        per_year -> per_month -> /11
        per_month -> per_year -> *11

        # Month has 30.42 days on average. About 7.9% discount to go monthly
        per_month -> per_day    -> /28
        per_day   -> per_month  -> *28

        # Day has 24h, give one for free
        per_day   -> per_hour   -> /23
        per_hour   -> per_day   -> /23


        Examples

        VPN @ 120CHF/y becomes
        - 10.91 CHF/month (130.91 CHF/year)
        - 0.39 CHF/day (142.21 CHF/year)

        VM @ 15 CHF/month becomes
        - 165 CHF/month (13.75 CHF/month)
        - 0.54 CHF/day (16.30 CHF/month)

        """


        if self.default_recurring_period == RecurringPeriod.PER_365D:
            if requested_period == RecurringPeriod.PER_365D:
                return self.recurring_price
            if requested_period == RecurringPeriod.PER_30D:
                return self.recurring_price/11.
            if requested_period == RecurringPeriod.PER_DAY:
                return self.recurring_price/11./28.

        elif self.default_recurring_period == RecurringPeriod.PER_30D:
            if requested_period == RecurringPeriod.PER_365D:
                return self.recurring_price*11
            if requested_period == RecurringPeriod.PER_30D:
                return self.recurring_price
            if requested_period == RecurringPeriod.PER_DAY:
                return self.recurring_price/28.

        elif self.default_recurring_period == RecurringPeriod.PER_DAY:
            if requested_period == RecurringPeriod.PER_365D:
                return self.recurring_price*11*28
            if requested_period == RecurringPeriod.PER_30D:
                return self.recurring_price*28
            if requested_period == RecurringPeriod.PER_DAY:
                return self.recurring_price
        else:
            # FIXME: use the right type of exception here!
            raise Exception("Did not implement the discounter for this case")


    def save(self, *args, **kwargs):
        # try:
        #     ba = BillingAddress.get_address_for(self.owner)
        # except BillingAddress.DoesNotExist:
        #     raise ValidationError("User does not have a billing address")

        # if not ba.active:
        #     raise ValidationError("User does not have an active billing address")


        # Verify the required JSON fields

        super().save(*args, **kwargs)



###
# Orders.

class Order(models.Model):
    """
    Order are assumed IMMUTABLE and used as SOURCE OF TRUST for generating
    bills. Do **NOT** mutate then!


    An one time order is "closed" (does not need to be billed anymore)
    if it has one bill record. Having more than one is a programming
    error.

    A recurring order is closed if it has been replaced
    (replaces__isnull=False) AND the ending_date is set AND it was
    billed the last time it needed to be billed (how to check the last
    item?)

    BOTH are closed, if they are ended/closed AND have been fully
    charged.

    Fully charged == fully billed: sum_of_order_usage == sum_of_bill_records

    """

    owner = models.ForeignKey(get_user_model(),
                              on_delete=models.CASCADE,
                              editable=True)

    billing_address = models.ForeignKey(BillingAddress,
                                        on_delete=models.CASCADE)

    description = models.TextField()

    product = models.ForeignKey(Product, blank=False, null=False, on_delete=models.CASCADE)
    config = models.JSONField()

    creation_date = models.DateTimeField(auto_now_add=True)
    starting_date = models.DateTimeField(default=timezone.now)
    ending_date = models.DateTimeField(blank=True, null=True)

    # FIXME: ensure the period is defined in the product
#    recurring_period = models.IntegerField(choices = RecurringPeriod.choices,
#                                           default = RecurringPeriod.PER_30D)
    recurring_period = models.ForeignKey(RecurringPeriod, on_delete=models.CASCADE, editable=True)

    one_time_price = models.DecimalField(default=0.0,
                                max_digits=AMOUNT_MAX_DIGITS,
                                decimal_places=AMOUNT_DECIMALS,
                                validators=[MinValueValidator(0)])

    recurring_price = models.DecimalField(default=0.0,
                                max_digits=AMOUNT_MAX_DIGITS,
                                decimal_places=AMOUNT_DECIMALS,
                                validators=[MinValueValidator(0)])

    currency = models.CharField(max_length=32, choices=Currency.choices, default=Currency.CHF)

    replaces = models.ForeignKey('self',
                                 related_name='replaced_by',
                                 on_delete=models.CASCADE,
                                 blank=True,
                                 null=True)

    depends_on = models.ForeignKey('self',
                                   related_name='parent_of',
                                   on_delete=models.CASCADE,
                                   blank=True,
                                   null=True)

    should_be_billed = models.BooleanField(default=True)

    @property
    def earliest_ending_date(self):
        """
        Recurring orders cannot end before finishing at least one recurring period.

        One time orders have a recurring period of 0, so this work universally
        """

        return self.starting_date + datetime.timedelta(seconds=self.recurring_period)

    @property
    def next_cancel_or_downgrade_date(self):
        """
        Return the next proper ending date after n times the
        recurring_period, where n is an integer that applies for downgrading
        or cancelling.
        """

        if self.recurring_period > 0:
            now = timezone.now()
            delta = now - self.starting_date

            num_times = ceil(delta.total_seconds() / self.recurring_period)

            next_date = self.starting_date + datetime.timedelta(seconds= num_times * self.recurring_period)
        else:
            next_date = self.starting_date

        return next_date

    def get_ending_date_for_bill(self, bill):
        """
        Determine the ending date given a specific bill
        """

        # If the order is quit, charge the final amount (?)
        if self.ending_date:
            this_ending_date = self.ending_date
        else:
            if self.earliest_ending_date > bill.ending_date:
                this_ending_date = self.earliest_ending_date
            else:
                this_ending_date = bill.ending_date

        return this_ending_date


    @property
    def count_billed(self):
        """
        How many times this order was billed so far.
        This logic is mainly thought to be for recurring bills, but also works for one time bills
        """

        return sum([ br.quantity for br in self.bill_records.all() ])

    def count_used(self, when=None):
        """
        How many times this order was billed so far.
        This logic is mainly thought to be for recurring bills, but also works for one time bills
        """

        if self.is_one_time:
            return 1

        if not when:
            when = timezone.now()

        # Cannot be used after it ended
        if self.ending_date and when > self.ending_date:
            when = self.ending_date

        return (when - self.starting_date) / self.default_recurring_period

    @property
    def all_usage_billed(self, when=None):
        """
        Returns true if this order does not need any further billing
        ever. In other words: is this order "closed"?
        """

        if self.count_billed == self.count_used(when):
            return True
        else:
            return False

    @property
    def is_closed(self):
        if self.all_usage_billed and self.ending_date:
            return True
        else:
            return False

    @property
    def is_recurring(self):
        return not self.recurring_period == RecurringPeriod.ONE_TIME

    @property
    def is_one_time(self):
        return not self.is_recurring

    def replace_with(self, new_order):
        new_order.replaces = self
        self.ending_date = end_before(new_order.starting_date)
        self.save()

    def update_order(self, config, starting_date=None):
        """
        Updating an order means creating a new order and reference the previous order
        """

        if not starting_date:
            starting_date = timezone.now()

        new_order = self.__class__(owner=self.owner,
                                   billing_address=self.billing_address,
                                   product=self.product,
                                   starting_date=starting_date,
                                   config=config)

        (new_order_one_time_price, new_order_recurring_price) = new_order.prices

        new_order.replaces = self
        new_order.save()

        self.ending_date = end_before(new_order.starting_date)
        self.save()


    def save(self, *args, **kwargs):
        if self.ending_date and self.ending_date < self.starting_date:
            raise ValidationError("End date cannot be before starting date")

        super().save(*args, **kwargs)


    def create_bill_record(self, bill):
        br = None

        if self.one_time_price != 0 and self.billrecord_set.count() == 0:
            br = BillRecord.objects.create(bill=bill,
                                           order=self,
                                           starting_date=self.starting_date,
                                           ending_date=self.starting_date,
                                           is_recurring_record=False)

        if self.recurring_price != 0:
            br = BillRecord.objects.filter(bill=bill, order=self, is_recurring_record=True).first()

            if br:
                self.update_bill_record_for_recurring_order(br, bill)

            else:
                br = self.create_new_bill_record_for_recurring_order(bill)

        return br

    def update_bill_record_for_recurring_order(self,
                                         bill_record,
                                         bill):
        """
        Possibly update a bill record according to the information in the bill
        """

        # If the order has an ending date set, we might need to adjust the bill_record
        if self.ending_date:
            if bill_record_for_this_bill.ending_date != self.ending_date:
                bill_record_for_this_bill.ending_date = self.ending_date

            else:
                # recurring, not terminated, should go until at least end of bill
                if bill_record_for_this_bill.ending_date < bill.ending_date:
                    bill_record_for_this_bill.ending_date = bill.ending_date

                bill_record_for_this_bill.save()

    def create_new_bill_record_for_recurring_order(self, bill):
        """
        Create a new bill record
        """

        last_bill_record = BillRecord.objects.filter(order=self, is_recurring_record=True).order_by('id').last()

        starting_date=self.starting_date

        if last_bill_record:
            # We already charged beyond the end of this bill's period
            if last_bill_record.ending_date >= bill.ending_date:
                return

            # This order is terminated or replaced
            if self.ending_date:
                # And the last bill record already covered us -> nothing to be done anymore
                if last_bill_record.ending_date == self.ending_date:
                    return

            starting_date = start_after(last_bill_record.ending_date)

        ending_date = self.get_ending_date_for_bill(bill)

        return BillRecord.objects.create(bill=bill,
                                         order=self,
                                         starting_date=starting_date,
                                         ending_date=ending_date,
                                         is_recurring_record=True)


    @property
    def prices(self):
        one_time_price = 0
        recurring_price = 0

        # FIXME: support amount independent one time prices
        # FIXME: support a base price
        # FIXME: adjust to the selected recurring_period

        if 'features' in self.product.config:
            for feature in self.product.config['features']:
                # FIXME: support optional features (?)
                if not feature in self.config['features']:
                    raise ValidationError(f"Configuration is missing feature {feature}")

                one_time_price += self.product.config['features'][feature]['one_time_price'] *  self.config['features'][feature]
                recurring_price += self.product.config['features'][feature]['recurring_price'] *  self.config['features'][feature]

        return (one_time_price, recurring_price)

    def save(self, *args, **kwargs):
        # Calculate the price of the order when we create it
        # IMMUTABLE fields -- need to create new order to modify them
        # However this is not enforced here...
        if self._state.adding:
            (self.one_time_price, self.recurring_price) = self.prices

        super().save(*args, **kwargs)


    def __str__(self):
        return f"{self.description} (order={self.id})"

class Bill(models.Model):
    """
    A bill is a representation of usage at a specific time
    """
    owner = models.ForeignKey(get_user_model(),
            on_delete=models.CASCADE)

    creation_date = models.DateTimeField(auto_now_add=True)
    starting_date = models.DateTimeField(default=start_of_this_month)
    ending_date = models.DateTimeField()
    due_date = models.DateField(default=default_payment_delay)


    billing_address = models.ForeignKey(BillingAddress,
                                        on_delete=models.CASCADE,
                                        editable=True,
                                        null=False)
    # FIXME: editable=True -> is in the admin, but also editable in DRF

    is_final = models.BooleanField(default=False)

    class Meta:
        constraints = [
            models.UniqueConstraint(fields=['owner',
                                            'starting_date',
                                            'ending_date' ],
                                    name='one_bill_per_month_per_user')
        ]

    def __str__(self):
        return f"Bill {self.owner}-{self.id}"

    def close(self):
        """
        Close/finish a bill
        """

        self.is_final = True
        self.save()

    @property
    def sum(self):
        bill_records = BillRecord.objects.filter(bill=self)
        return sum([ br.sum for br in bill_records ])

    @classmethod
    def create_bills_for_all_users(cls):
        """
        Create next bill for each user
        """

        for owner in get_user_model().objects.all():
            cls.create_next_bills_for_user(owner)

    @classmethod
    def create_next_bills_for_user(cls, owner, ending_date=None):
        """
        Create one bill per billing address, as the VAT rates might be different
        for each address
        """

        bills = []

        for billing_address in BillingAddress.objects.filter(owner=owner):
            bills.append(cls.create_next_bill_for_user_address(billing_address, ending_date))

        return bills

    @classmethod
    def get_or_create_bill(cls, billing_address):
        last_bill = cls.objects.filter(billing_address=billing_address).order_by('id').last()

        all_orders = Order.objects.filter(billing_address=billing_address).order_by('id')
        first_order = all_orders.first()

        bill = None
        ending_date = None

        # Get date & bill from previous bill, if it exists
        if last_bill:
            if not last_bill.is_final:
                bill = last_bill
                starting_date = last_bill.starting_date
                ending_date = bill.ending_date
            else:
                starting_date = last_bill.ending_date + datetime.timedelta(seconds=1)
        else:
            # Might be an idea to make this the start of the month, too
            if first_order:
                starting_date = first_order.starting_date
            else:
                starting_date = timezone.now()

        if not ending_date:
            ending_date = end_of_month(starting_date)

        if not bill:
            bill = cls.objects.create(
                owner=billing_address.owner,
                starting_date=starting_date,
                ending_date=ending_date,
                billing_address=billing_address)


        return bill

    @classmethod
    def create_next_bill_for_user_address(cls,
                                          billing_address,
                                          ending_date=None):

        """
        Create the next bill for a specific billing address of a user
        """

        owner = billing_address.owner

        all_orders = Order.objects.filter(owner=owner,
                                          billing_address=billing_address).order_by('id')

        bill = cls.get_or_create_bill(billing_address)

        for order in all_orders:
            order.create_bill_record(bill)

        return bill

    # @classmethod
    # def create_bill_records_for_recurring_orders(cls, bill):
    #     """
    #     Create or update bill records for recurring orders for the
    #     given bill
    #     """

    #     owner = bill.owner
    #     billing_address = bill.billing_address

    #     all_orders = Order.objects.filter(owner=owner,
    #                                       billing_address=billing_address).order_by('id').exclude(recurring_period=RecurringPeriod.ONE_TIME)

    #     for order in all_orders:
    #         bill_record_for_this_bill = BillRecord.objects.filter(bill=bill,
    #                                                               order=order).first()
    #         if bill_record_for_this_bill:
    #             cls.update_bill_record_for_this_bill(bill_record_for_this_bill,
    #                                                  order,
    #                                                  bill)

    #         else:
    #             cls.create_new_bill_record_for_this_bill(order, bill)


    @classmethod
    def create_next_bill_for_user_address_old(cls,
                                          owner,
                                          billing_address,
                                          ending_date=None):
        """
            Filtering ideas (TBD):
            If order is replaced, it should not be added anymore if it has been billed "last time"
            If order has ended and finally charged, do not charge anymore

           Find out the last billrecord for the order, if there is none, bill from the starting date
        """

        last_bill = cls.objects.filter(owner=owner, billing_address=billing_address).order_by('id').last()

        # it is important to sort orders here, as bill records will be
        # created (and listed) in this order
        all_orders = Order.objects.filter(owner=owner,
                                          billing_address=billing_address).order_by('id')
        first_order = all_orders.first()

        bill = None
        ending_date = None

        # Get date & bill from previous bill, if it exists
        if last_bill:
            if not last_bill.is_final:
                bill = last_bill
                starting_date = last_bill.starting_date
                ending_date = bill.ending_date
            else:
                starting_date = last_bill.ending_date + datetime.timedelta(seconds=1)
        else:
            if first_order:
                starting_date = first_order.starting_date
            else:
                starting_date = timezone.now()

        if not ending_date:
            ending_date = end_of_month(starting_date)

        if not bill:
            bill = cls.objects.create(
                owner=owner,
                starting_date=starting_date,
                ending_date=ending_date,
                billing_address=billing_address)

        for order in all_orders:
            if order.is_one_time:
                # this code should be ok, but needs to be double checked
                if order.billrecord_set.count() == 0:
                    br = BillRecord.objects.create(bill=bill,
                                                   order=order,
                                                   starting_date=order.starting_date,
                                                   ending_date=order.ending_date)

            else: # recurring orders
                bill_record_for_this_bill = BillRecord.objects.filter(bill=bill,
                                                                      order=order).first()

                # This bill already has a bill record for the order
                # We potentially need to update the ending_date if the ending_date
                # of the bill changed.
                if bill_record_for_this_bill:
                    # we may need to adjust it, but let's do this logic another time

                    # If the order has an ending date set, we might need to adjust the bill_record
                    if order.ending_date:
                        if bill_record_for_this_bill.ending_date != order.ending_date:
                            bill_record_for_this_bill.ending_date = order.ending_date

                    else:
                        # recurring, not terminated, should go until at least end of bill
                        if bill_record_for_this_bill.ending_date < bill.ending_date:
                            bill_record_for_this_bill.ending_date = bill.ending_date


                    bill_record_for_this_bill.save()

                else: # No bill record in this bill for this order yet
                    # Find out when billed last time
                    last_bill_record = BillRecord.objects.filter(order=order).order_by('id').last()

                    # Default starting date
                    this_starting_date=order.starting_date

                    # Skip billing again, if we have been processed for this bill duration
                    if last_bill_record:
                        if last_bill_record.ending_date >= bill.ending_date:
                            continue

                        # If the order ended and has been fully billed - do not process it
                        # anymore
                        if order.ending_date:
                            if last_bill_record.ending_date == order.ending_date:
                                # FIXME: maybe mark order for not processing anymore?
                                # I imagina a boolean column, once this code is stable and
                                # verified
                                continue

                            # Catch programming bugs if the last bill_record was
                            # created incorrectly - should never be entered!
                            if order.ending_date < last_bill_record.ending_date:
                                raise ValidationError(f"Order {order.id} ends before last bill record {last_bill_record.id}")

                        # Start right after last billing run
                        this_starting_date = last_bill_record.ending_date + datetime.timedelta(seconds=1)


                    # If the order is already terminated, use that date instead of bill date
                    if order.ending_date:
                        this_ending_date = order.ending_date
                    else:
                        if order.earliest_ending_date > bill.ending_date:
                            this_ending_date = order.earliest_ending_date
                        else:
                            # bill at maximum for this billing period
                            this_ending_date = bill.ending_date

                    # And finally create a new billrecord!
                    br = BillRecord.objects.create(bill=bill,
                                                   order=order,
                                                   starting_date=this_starting_date,
                                                   ending_date=this_ending_date)




        return bill


class BillRecord(models.Model):
    """
    Entry of a bill, dynamically generated from an order.
    """

    bill = models.ForeignKey(Bill, on_delete=models.CASCADE)
    order = models.ForeignKey(Order, on_delete=models.CASCADE)

    creation_date = models.DateTimeField(auto_now_add=True)
    starting_date = models.DateTimeField()
    ending_date = models.DateTimeField()

    is_recurring_record = models.BooleanField(blank=False, null=False)

    @property
    def quantity(self):
        """ Determine the quantity by the duration"""
        if not self.is_recurring_record:
            return 1

        record_delta = self.ending_date - self.starting_date

        return record_delta.total_seconds()/self.order.recurring_period

    @property
    def sum(self):
        if self.is_recurring_record:
            return self.order.recurring_price * Decimal(self.quantity)
        else:
            return self.order.one_time_price

    def __str__(self):
        if self.is_recurring_record:
            bill_line =  f"{self.starting_date} - {self.ending_date}: {self.quantity} x {self.order}"
        else:
            bill_line =  f"{self.starting_date}: {self.order}"

        return bill_line

    def save(self, *args, **kwargs):
        if self.ending_date < self.starting_date:
            raise ValidationError("End date cannot be before starting date")

        super().save(*args, **kwargs)


# # Sample products included into uncloud
# class SampleOneTimeProduct(models.Model):
#     """
#     Products are usually more complex, but this product shows how easy
#     it can be to create your own one time product.
#     """

#     default_recurring_period = RecurringPeriod.ONE_TIME

#     ot_price = models.IntegerField(default=5)

#     @property
#     def one_time_price(self):
#         return self.ot_price

# class SampleRecurringProduct(models.Model):
#     """
#     Products are usually more complex, but this product shows how easy
#     it can be to create your own recurring fee product.
#     """

#     default_recurring_period = RecurringPeriod.PER_30D

#     rc_price = models.IntegerField(default=10)

#     @property
#     def recurring_price(self):
#         return self.rc_price

# class SampleRecurringProductOneTimeFee(models.Model):
#     """
#     Products are usually more complex, but this product shows how easy
#     it can be to create your own one time + recurring fee product.
#     """

#     default_recurring_period = RecurringPeriod.PER_30D

#     ot_price = models.IntegerField(default=5)
#     rc_price = models.IntegerField(default=10)

#     @property
#     def one_time_price(self):
#         return self.ot_price

#     @property
#     def recurring_price(self):
#         return self.rc_price