Restricting single Rating on each Book by each User : Django ORM

问题

Edited Title : Limit multiple "Many to One" fields into "One to One" association : Django

We've Book, User and Rating as Django model.

Each Book has many Ratings
Each Rating has one Book
Each User has many Ratings
Each Rating has one User

For Book

class Book(models.Model):
    isbn = models.CharField(max_length=32)
    title = models.CharField(max_length=256)

    def __str__(self):
        return self.title

For Book Rating

class BookRating(models.Model):
    book = models.ForeignKey(Book, on_delete=models.CASCADE)
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    rating = models.SmallIntegerField(choices=[(i, i) for i in range(1, 6)])

    def __str__(self):
        return self.rating

Problem Statement

How can I ensure that each User has atmost one rating on each book?

回答1:

Just do

class BookRating(models.Model):
    book = models.ForeignKey(Book, on_delete=models.CASCADE)
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    rating = models.SmallIntegerField(choices=[(i, i) for i in range(1, 6)], default=1)

    class meta:
        unique_together = ('book','user')

Your models do implement a many to many relationship, however you are not getting full access to the django ManyToMany functionality. I recommend you do something like this:

class Book(models.Model):
    isbn = models.CharField(max_length=32)
    title = models.CharField(max_length=256)
    ratings = models.ManyToManyField(User,through='BookRating')

When you do this your BookRating can remain unchanged, but the small change to the Book model gives you full access to the api described here: https://docs.djangoproject.com/en/1.10/topics/db/examples/many_to_many/

What's interesting is that modifying the Book model as described above does not make any changes to your table structures in your database. They remain unchanged. It's simply a matter of unlocking the api.

来源：https://stackoverflow.com/questions/41703940/restricting-single-rating-on-each-book-by-each-user-django-orm

标签

python

django

django-models

django-orm