models.py

class Contract:

  def total_paid(self, at=None):
		'''Returns total amount paid due to this Contract'''

		# Filter
		params = {}
		if at:
			params.update({"date_paid__lte": at})

		return self.payments.filter(**params).aggregate(Sum('amount'))['amount__sum'] or 0.0

  def balance(self, in_months=False, at=None):
		'''Looks at how much has been paid for this Contract and determines if there is any balance owed by (-ve) / owed to (+ve) the Member'''
		balance = 0
		duration_in_months = 0

		if at and self.start > at:
			return 0  # There is no balance if this Contract started after given date


		# Calculate number of months Contract has been in effect, ie. not Terminated
		if at and (self.end and self.end > at):
			print "2"
			duration_in_months += self.__month_diff(at, self.start)
		elif self.end or (self.end and at and self.end < at):
			print "1"
			duration_in_months += self.__month_diff(self.end, self.start)
		else:
			print "3"
			duration_in_months += self.__month_diff(datetime.date.today(), self.start)

		balance = self.total_paid(at=at) - (self.tier.fee * duration_in_months)

		if in_months:
			return balance / self.tier.fee

		else:
			return balance

Prepayment.py

from hado.models import  *                                                  
import datetime                                                              
f = open("prepayments.csv", "w+")                                            
import csv                                                                   
ppcsv = csv.DictWriter(f, ["user", "email", "balance"])                      
ppcsv.writeheader()                               
total = 0                           
users = User.objects.all().order_by('first_name')                            
for u in users:                                                              
  for c in u.contracts.exclude(start__gte=datetime.date(2010, 10, 1)):       
    balance = c.balance(at=datetime.date(2010, 9, 30))                       
    if balance > 0:                                                          
      ppcsv.writerow({"user":u.get_full_name(), "email":u.email, "balance":  balance}) 
      total += balance