Django OneToOneField, ManyToManyField, Foreign Key

Question

I've read many posts on what OneToOneField, ManyToManyField, and Foreign Key are but they aren't very clear. I am very new to Django and python programming, currently trying to develop models. Can someone explain to me in simple language, preferably with example, what they each are?

Answer 1

Imagine a database, which stores your book collection:

from django.db import models

class Place(models.Model):
    address = models.CharField(max_length=50)
    country = models.CharField(max_length=50)

class Publisher(models.Model):
    name = models.CharField(max_length=30)
    place = models.OneToOneField(Place, primary_key=True)

class Author(models.Model):
    first_name = models.CharField(max_length=30)
    last_name = models.CharField(max_length=40)

class Book(models.Model):
    title = models.CharField(max_length=100)
    publisher = models.ForeignKey(Publisher)
    authors = models.ManyToManyField(Author)

One-to-many/Foreign Key

Every Book has one Publisher, but a Publisher might have published multiple books. Therefore they are in a one-to-many (book-to-publisher) relationship.

One-to-one

Every Publisher is located in one Place, and every Place can only hold one Publisher. Therefore they are in a one-to-one relationship. You could just have well have put the Place information (address and country) with the Publisher model in one table, but sometimes it is preferred to have seperate models. For example, if you do not know the Place for every Publisher, you don't need to take up a lot of space with empty rows.

Many-to-many

Every Book also has one or more Authors. However, an Author might have written multiple Books, so they are in a many-to-many relationship.

If you still need some guidance, I suggest taking a look at the model chapter of the Django Book.