Associate addditional value to a manytomanyfield
Associate addditional value to a manytomanyfield
I have 2 models book and favorite
class Book(models.Model):
title = models.CharField(max_length=200)
class Favorite(models.Model):
user = models.OneToOneField(User, on_delete=models.CASCADE)
favorites = models.ManyToManyField(Book, related_name='favorited_by')
Here a user has its own sets of favorite books. I want to associate an additional variable to each selected favorite book but how can this be done?
through
But your modeling is actually quite strange to begin with, you use a
OneToOne to a user?– Willem Van Onsem
Jul 2 at 19:29
OneToOne
what is the other way? I'm new to django.
– Ahmed
Jul 2 at 19:34
1 Answer
1
You can define a through model: a model that is used to write many-to-many relations. But your modeling in general is a bit strange. Why do you use a OneToOneField to a User here? Isn't a Favorite a single favorite (thus the fact that a single user likes a single book:
through
OneToOneField
User
Favorite
from django.conf import settings
class Book(models.Model):
title = models.CharField(max_length=200)
users = models.ManyToManyField(settings.AUTH_USER_MODEL, through='Favorite')
class Favorite(models.Model):
user = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
book = models.ForeignKey(Book, on_delete=models.CASCADE)
additional = models.IntegerField()
We thus model it as a three models:
+-------+ 1 M +------------+ N 1 +------+
| Book |-------| Favorite |--------| User |
+-------+ +------------+ +------+
| title | | additional |
+-------+ +------------+
Now Favorite is thus seen as the fact that a User liked a Book. Not as one-to-one model instance attached to a User that will contain all favorites.
Favorite
User
Book
User
For more, see the documentation on the through parameter [Django-doc].
through
I am implementing in a way that user can select multiple favorites
– Ahmed
Jul 2 at 19:47
@Ahmed: well that is here perfectly possible, since every favorite a user has, is a single
Favorite instance for that user. It is rather weird that you use a model called Favorite to store *multiple Favorites in. In your implementation, the Favorite has not much added value, since your Favorite basically acts like a proxy object to a User.– Willem Van Onsem
Jul 2 at 19:49
Favorite
Favorite
Favorite
Favorite
Favorite
User
Typically using such shallow models with only a
OneToOne field are not good since (a) they create tables that only add data duplication (which is already an anti-pattern), (b) it will make querying more complex, and (c) and it is less efficient.– Willem Van Onsem
Jul 2 at 19:52
OneToOne
Oh yes it is working Thankyou.
– Ahmed
Jul 2 at 19:54
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By specifying a
throughmodel...– Willem Van Onsem
Jul 2 at 19:27