Help creating model for Django app

I'm trying to make a childcare administration app with Django but I've some problems with the payments code.

Each kid has to pay monthly 10 times a year. These payments have some particularities:

• Some kids could pay a different amount of money depending on the economical situation of the parents.

• The amount of the payments could change over the year.

• The payments could be payed in different oportunities.

• Workshops and food are added to the total value of the payment(they're optional). 开发者_开发知识库

I'm having problem in order to model this app. I was thinking about creating a kid model and a payment model:

class Kid(models.Model):
    food = models.BooleanField()
    workshop = models.BooleanField()
    special_price = models.DecimalField(blank=True)

class Payment(models.Model):
    kid = models.ForeignKey(Kid)
    date = models.DateField() 
    amount = models.DecimalField()`

Suppose I want to know in a given date how much moneya kid(the parents) have to pay. I don't know how to solve the changing values problem(the food and workshop values also could change over the year).

Hope you understood me, Thanks. mF.

No matter how your final models will look like, assuming that you will keep Payment model, you could add to it two fields:

    payment_date = model.DateField()
    already_paid = models.BooleanField()

Then, to get overdue payments, you will be able to make a query:

    Payment.objects.filter(payment_date__lte=datetime.date.today(), already_paid=False)

Having those Payment objects, you will be able to calculate the exact amount.

I would make the payment calculated off of the attributes of the child so by updating the child the payment would change. The payment would not be a entry in the database until it had been paid, the payment would be calculated each time the user requests it.

class Child(object):
   def __init__(self,income,paydate,meals=False,workshops=False):
      self.income = income
      self.paydate = paydate
      ...

I would also have a pricing rules model that in combination with the child object would give you the payment amount. Again updating the PricingRules would change the payment.

class PricingRules(object):
   def __init__(self,income_scale,paydate,meal_price,workshop_price,num_payments):
      ...

I would avoid having a PricingRules object for each Child. There would then be the Payment object

class Payment(object):
   def __init__(self,amount,date_paid):
      ...

I've avoided showing any code that is specific to the ORM being used, I believe the design should stand on its own, and I also like to keep the possibility open of using the objects without the ORM. Using constructors is also my personal style, I like knowing that when an object is created from a class that it will be in a state that is well defined and stable. Without more detail regarding the business logic I have more questions than answer beyond this point.