Creating "classes" with Django

Question

I'm just learning Django so feel free to correct me in any of my assumptions. I probably just need my mindset adjusted.

What I'm trying to do is creating a "class" in an OOP style. For example, let's say we're designing a bunch of Rooms. Each Room has Furniture. And each piece of Furniture has a Type and a Color. What I can see so far is that I can have

class FurnitureType(models.Model):
    name = models.CharField(max_length=200)

class FurnitureColor(models.Model):
    name = models.CharField(max_length=50)

class FurniturePiece(models.Model):
    type = models.ForeignKey(FurnitureType)
    color = models.ForeignKey(FurnitureColor)
    sqft = models.IntegerField()
    name = models.CharField(max_length=200)

class Room(models.Model):
    name = models.CharField(max_length=200)
    furnitures = models.ManyToManyField(FurniturePiece)

The problem is that each FurniturePiece has to have a unique name if I'm picking it out of the Django admin interface. If one person creates "Green Couch" then no one else can have a "Green Couch". What I'm wondering is if a) I need to learn more about Django UI and this is the right way to design this in Django or b) I have a bad design for this domain

The reason I want Furniture name to be unique is because 10 people could create a "Green Couch" each with a different sqft.

Answer 1

Why does each FurniturePiece need to have a unique name? It seems to me that if you remove that constraint everything just works.

(as an aside you seem to have accidentally dropped the models.Model base class for all but the Room model).

Answer 2

I don't get the problem with unique name. You can just specify it to be unique:

class FurniturePiece(models.Model):
    type = models.ForeignKey(FurnitureType)
    color = models.ForeignKey(FurnitureColor)
    sqft = models.IntegerField()
    name = models.CharField(max_length=200, unique=True)

I don't know whether you have to learn about Django UI or not. I guess you have to learn how to define models. The admin interface is just a generated interface based on your models. You can change the interface in certain aspects without changing the models, but besides that, there is less to learn about the admin interface.

I suggest you follow a tutorial like the djangobook, to get a good start with Django.

---

I think, the problem that you have is not how to use Django but more that you don't know how to model your application in general.

• First you have to think about which entities do yo have (like Room, Furniture, etc.).
• Then think about what relations they have.
• Afterwards you can model them in Django. Of course in order to do this you have to know how to model the relations. The syntax might be Django specific but the logical relations are not. E.g. a many-to-many relation is not something Django specific, this is a term used in databases to express a certain relationship.

Djangos models are just abstraction of the database design below.

---

E.g you specified a many-to-many relationship between Room and FurniturePiece.
Now the question: Is this what you want? It means that a piece of furniture can belong to more than one room. This sounds strange. So maybe you want to model it that a piece of furniture only belongs to one room. But a room should still have several pieces of furniture. We therefore define a relationship from FurniturePiece to Room.

In Django, we can express this with:

class FurniturePiece(models.Model):
    room = models.ForeignKey(Room)
    type = models.ForeignKey(FurnitureType)
    color = models.ForeignKey(FurnitureColor)
    sqft = models.IntegerField()
    name = models.CharField(max_length=200)

---

Maybe you should first learn about relational databases to get the basics before you model your application with Django.

It might be that this not necessary in order to create an application in Django. But it will definitely help you to understand whats going on, for every ORM not just Django's.

Answer 3

This is how I would do it:

class Room(models.Model):
    name = models.CharField(max_length=255)
    pieces = models.ManyToManyField('FurniturePiece')

class FurniturePiece(models.Model):
    itemid = models.CharField(max_length=20, unique=True)  # This is what I would require to be unique.
    name = models.CharField(max_length=255)
    type = models.ForeignKey('FurnitureType')  # Note I put 'FurnitureType' in quotes because it hasn't been written yet (coming next).
    color = models.ForeignKey('FurnitureColor')  # Same here.
    width_in_inches = models.PositiveIntegerField()
    length_in_inches = models.PositiveIntegerField()

    # Next is the property decorator which allows a method to be called without using ()
    @property
    def sqft(self):
        return (self.length_in_inches * self.width_in_inches) / 144  # Obviously this is rough.


class FurnitureType(models.Model):
    name = models.CharField(max_length=255)

class FurnitureColor(models.Model):
    name = models.CharField(max_length=255)

Envision objects as real life objects, and you'll have a deeper understanding of the code as well. The reason for my sqft method is that data is best when normalized as much as possible. If you have a width and length, then when somebody asks, you have length, width, sqft, and if you add height, volume as well.