creating model's related objects

Hi everyone,
I was wondering about the best practice of creating related model objects once a specific model object is created.
for example, once a user is registered (i.e creating a User model object with the entry details), I would like to automatically create user books list object as well and have it be associated with that user object.

based on my limited knowledge I know there are many ways to do this, but what is the best one, which ones are the overkill?

  • using post_save signals
  • overriding save() function on the user model, to create the other objects.
  • calling the books list object constructor just after the user constructor was called?

Many Thanks in Advance

First, I never recommend signals as anything other than a last resort. There are just enough “got’chas” that I personally recommend against them unless you have no other option.

Beyond that, I’d say the question really comes down to how the data is being entered into the system. You mentioned creating a book list object, but that doesn’t make sense to me without having a list of books to assign.

So, my opinion is that the answer would depend upon factors beyond the information provided in your original post, including more details about the models involved.

Apologies, I should have explained more to be more clear.
Okay so the system is the following:

  • users can register in a website (as a books lending website)
  • users can create objects of books they can lend to others.
  • users can mark books they are interested to borrow in a watchlist (books list that I have mentioned in the original post)

obviously this would have other details of the processes entailed, and other functionalities, the idea of this system is just an example to explain my question.

I hope this is clearer, and thank you for your reply.

When you say “automatically create” the books once the User is created, what do you actually mean? Do you just want to populate some test data so you can test things out as you go? Or will the user be uploading a list of books somehow?

What is providing the data for this automatic creation?

“automatic” as in simultaneous creation for the books list (or in another term like watchlist) once the User object is created, without having an extra step from the user, but it’s registration.

It might be clearer if you actually post the model’s you’re working with. I’m not seeing where a book list object “fits” into the desired system functionality. Typically, you would have a single model for books, and then create a relationship between the books and the users.

I’m actually just confused where this book list would come from when you create the user? Do they input a list of books when they enter their profile info?

I don’t really see how you can add a list of the user’s books “automatically” without them telling you what those books are …

here are the initial models:

class User(AbstractUser):

class Category(models.Model):
    name = models.CharField(max_length=64)

class Book(models.Model):

    owner = models.ForeignKey(User, on_delete=models.CASCADE, related_name="books")
    title = models.CharField(max_length=64)
    description = models.CharField(max_length=255)
    category = models.ForeignKey(Category, on_delete=models.RESTRICT, blank=True, null=True)
    image = models.ImageField(upload_to="books/images/", default="", blank=True)

class Watchlist(models.Model):
    owner = models.OneToOneField(User, on_delete=models.CASCADE, related_name="watchlist")
    books = models.ManyToManyField(Book, blank=True, related_name="watchlist_books")

reflecting on your comment, I am wondering if my design is faulty, and it is best practice to add a many-to-many field in the Book class representing all of the users that showed interest in a specific book (i.e. added the book to their watchlist)?

That was the general thought behind my comment. I didn’t say so explicitly because I wasn’t sure if this was your real models/design or a representative example being used to explain a much different application.

But yes, my general suggestion in this case would be to put a ManyToMany field in either your custom user or your Book model. (It really doesn’t matter which)

Also, if you’re going to want to track additional attributes on that relationship, you might want to initially design it with the “through” attribute, even if that “through” table only has the two foreign keys to start.
(You might want to track something like “preference” to represent a priority, or a “has read” to indicate that they’ve already read the book - anything that provides additional detail of the relationship between User and Book.)

wow, I thought my knowledge of Many-to-Many relationships and even using “through attribute” was sufficient enough after working on many projects. It makes total sense, and it’s even quite obvious, I am surprised I missed this :sweat_smile: :sweat_smile:
Thank you very much for your help and for pointing that out.

Best regards,