I have few problems with Django models (relations mostly), but I'll start from code than make desciption and put questions...

Django models and relations in short (very simplified) version:

# Shorten version of services
SERVICES = [
 ("TC", "Tires Cleaning"),
 ("EC", "Exterior Cleaning"),
 ("IC", "Interior Cleaning"),
 ("CW", "Cleaning & Washing - full service"),
 ("WX", "Application of wax"),
]


class Client(models.Model):
    name = models.CharField(max_length=35)
    surname = models.CharField(max_length=35)
    car = models.CharField(max_length=25)


class SingleService(models.Model):
    service_name = models.CharField(max_length=35)    
    service_type = models.CharField(max_length=1, choices=SERVICES, default="CW")
    price = models.IntegerField()


class SetOfServices(models.Model):
    set_of_services = ????
    price_of_set = ???


class Purchase(models.Model):
    client = OneToOneField(client)  
    order = models.ForeignKey(SetOfServices, on_delete=models.CASCADE, blank=True, null=True)
    price = ???

I want to make small project (Car Wash) with 8 django models. Client is coming to car wash and I want to store its name/ surname and car make/model/type etc in DB for future refference (clients coming back will get discount and free coffee). Then I have SingleService model with about 20-30 services - they have different name and price because of diferernt tasks / time to make and options / extra features. SetOfServices model must consist of few single services (custom, but customized from Django admin panel, probably only once, at setup / configuration time). So it must be set of few services (I don't want client to buy only one / single service!), like:

Example SET 1: Tires cleaning Interior cleaning

Example SET 2: Exterior cleaning Application of wax

....and so on. I want to join few services in one set of services client can buy (with some discount - for example).

The last model (from above code, I plan to expand this project) is Purchase where Client orders one SetOfServices and pay some money. The problems I encountered is choosing right relation for set_of_services field and how to calculate overall price of this set (price_of_set field). Should I use ForeignKey (One to Many) relation in set_of_service field?

How to divide Django models (when project expands) into few tables in database (I am using PostgreSQL)? Could you give me refference or example code for it? I know we use db_table in Meta description of models, but how the relations change??

CodePudding user response：

The question you want to ask yourself is:

• Does one SetOfServices contain multiple SingleServices --> YES
• Does one SingleService show up in multiple different SetOfServices? --> YES

If your result to these questions is two times YES the relationship is a many-to-many.

class SetOfServices(models.Model):
    set_of_services = models.ManyToManyField(SingleService)
    # price_of_set --> does this really need to be stored in database?
   
    @property
    def price_of_set(self):
        return sum([service.price for service in self.set_of_services]

Now you want to ask yourself:

• Does a Purchase allow buying multiple SetOfServices (this is dependent on your choice)? --> NO
• Is a SetOfServices bought by multiple Purchases? --> YES

YES YES --> ManyToMany
NO YES --> OneToMany --> ForeignKey

class Purchase(models.Model):
    client = OneToOneField(client)  
    order = models.ForeignKey(SetOfServices, on_delete=models.CASCADE, blank=True, null=True)
    # same advice as above for price = ???

    @property
    def price(self):
        return self.order.price_of_set

To be honest I feel like your setup of models is already kinda stable. I can't tell you how to divide models at runtime of a project. I've never done it.

Let me know how it goes!