What is related_name used for?

Gareth D.

The problem

You’ve seen related_name defined in your Django models and you’re not sure what it does, or you want to use related_name and you’re not sure how.

The solution

The related_name field lets you access foreign keys defined in your Django models backwards, and override the default <model>_set field that Django creates for convenient access to your linked models.

For example, usually if you want to find all books written by a specific author, you have to look in a Books model and filter by author, using a query like Book.objects.filter(author=some_author). Sometimes it’s more convenient to start from the Author model and find all books by that author, using some_author.book_set.all()

The related_name field lets you define what to call this backwards link. By default, Django already creates this for you using the name of the foreign key and the _set suffix. Let’s use the following two models as an example.

from django.db import models

class Author(models.Model):
    name = models.CharField(max_length=200)

    def __repr__(self):
        return self.name

class Book(models.Model):
    author = models.ForeignKey(Author, on_delete=models.CASCADE)
    title = models.CharField(max_length=200)

    def __repr__(self):
        return f"{self.title} by {self.author.name}"

Let’s say we have only two authors in our database: Alex Smith and Sam Jones. Alex has written two books and Sam has written one. In a Django shell, we could run:

>>> from booktracker.models import Book, Author
>>> Author.objects.all()
<QuerySet [Alex Smith, Sam Jones]>
>>> Book.objects.all()
<QuerySet [How Django Works by Alex Smith, How to Sell Django Books by Alex Smith, What Alex Got Wrong by Sam Jones]>

The traditional way to get all the books written by Alex would be to import both the book and the author models, do a query for the author of interest, and then pass in that author as a filter when querying for books. This could be done as follows.

>>> from booktracker.models import Book, Author
>>> alex = Author.objects.get(name="Alex Smith")
>>> alex_books = Book.objects.filter(author=alex)
>>> alex_books
<QuerySet [How Django Works by Alex Smith, How to Sell Django Books by Alex Smith]>

And if you’re used to writing SQL statements, this is probably intuitive. The book table has a foreign key link to the author table, so each book “knows” who its author is, but authors don’t “know” what books they wrote. If you’re a human, it’s probably not intuitive and you’d prefer to deal only with the Author object and get a list of books belonging to that author. With Django, you can do look-ups in reverse as follows.

>>> from booktracker.models import Author
>>> alex = Author.objects.get(name="Alex Smith")
>>> alex.book_set.all()
<QuerySet [How Django Works by Alex Smith, How to Sell Django Books by Alex Smith]>

Which gets the same results using less code, and we don’t even have to import our Books model.

The book_set is the automatic related_name that Django assigned to our Author model when we created the Book model with a ForeignKey linking it to Author. If you want to use a different name, you can customize it by setting related_name. For example, if you change the definition of the Book model to:

class Book(models.Model):
    author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name="books")
    title = models.CharField(max_length=200)

    def __repr__(self):
        return f"{self.title} by {self.author.name}"

Then you can get all the books for a specific author using alex.books.all() instead of alex.book_set.all(), which is more intuitive still (note that this is a database change, so you’ll need to run migrations again after making it in the models.py file).

>>> from booktracker.models import Author
>>> alex = Author.objects.get(name="Alex Smith")
>>> alex.books.all()
<QuerySet [How Django Works by Alex Smith, How to Sell Django Books by Alex Smith]>

Disable related_name

If you want to disable this backwards link completely (for example, to save a bit of space in the database index that Django needs to make this work), you can set related_name="+". After doing this, the book_set relation will no longer exist.

>>> from booktracker.models import Author
>>> alex = Author.objects.get(name="Alex Smith")
>>> alex.book_set.all()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
AttributeError: 'Author' object has no attribute 'book_set'