Question

Preface: I have two models (Product and UserProfile) and I would to implement a mine comments system. The comment is related to an object, Product or UserProfile.

class Product(models.Model):
    name = models.CharField(max_length = 40)
    comments = models.ManyToMany(Comment)

class UserProfile(models.Model):
    user = models.ForeignKey(User, unique = True)
    comments = models.ManyToMany(Comment)

class Comment(models.Model):
    text = models.TextField()
    author = models.ForeignKey(User)
    timestamp = models.DateTimeField(auto_now_add = True)

Is it correct the logic under these models? I’m doubtful, because this way means a Product can has many comments (and it’s correct) but also a Comment can has many products (I don’t think it’s correct).

It isn’t?

Answer 1

Your comment should have have a ForeignKey to the UserProfile or Product i.e. A single comment can only belong to a single product or user profile, but a user profile/product can have many different comments

def Comment(models.Model):
    profile = models.ForeignKey(UserProfile)
    product = models.ForeignKey(Profile)

Obviously this isn’t ideal as there are two relationships you need to manage, and sometimes you will only want to use one etc.

To get over this, you can use a Generic Foreign Key:

https://docs.djangoproject.com/en/dev/ref/contrib/contenttypes/#generic-relations

this allows you to link a comment to any type of content (products, user profiles and more) without having to specify the models up front

def Comment(models.Model):
    ...
    content_type = models.ForeignKey(ContentType)
    object_id = models.PositiveIntegerField()
    content_object = generic.GenericForeignKey('content_type', 'object_id')