Traversing Django ORM chains

This is not something I learned today, but something I encountered a few times while working within django’s ORM:

Depending on the direction of a relationship between models, the casing of relationship-lookup changes.

Naturally I decided to try to find something in the documentation and write down this pattern. Let’s assume the following models.

class Author(models.Model):
    name = models.CharField(max_length=100)

class Book(models.Model):
    title = models.CharField(max_length=100)
    author = models.ForeignKey(Author, on_delete=models.CASCADE)

class RentedBookRecord(models.Model):
  book = models.ForeignKey(Book, on_delete=models.CASCADE)
  rented_by = models.CharField(max_length=100)

If the relationship is a direct relationship (like a foreign-key) to a different model we can use the name of the field as the filter key.

books_by_author = Book.objects.filter(author__name="Johnny Silverhand")

If the relationship is an inverse relationship, the lowercase model name becomes the key.

It works backwards, too. While it can be customized, by default you refer to a “reverse” relationship in a lookup using the lowercase name of the model. ¹

rented_by_reader = Book.objects.filter(rentedbookrecord__rented_by="V")