Question

In my models I have the classes Book and Category defined like this:

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

class Book(models.Model):
    title = models.CharField()
    categories = models.ManyToManyField(Category)

What I want is the set of Category instances that are referenced in the categories field of a given queryset of Book instances.

I realize I can iterate through the queryset of books and gather the categories for each book but this seems inefficient to me as this could be stated in a single SQL query:

SELECT DISTINCT name
FROM myapp_book_categorys JOIN myapp_category ON myapp_book_categorys.category_id=myapp_category.id
WHERE myapp_book_categorys.book_id IN 
    (SELECT id FROM myapp_book WHERE ...);

Is the raw SQL the right way to go or there is a higher level solution comparable in efficiency?

Answer 1

Edit: Okay, I didn’t have a ManyToManyField to test on before so I was guessing. New code!

books = Book.objects.filter(title__contains="T")
categories = Category.objects.filter(book__in=books).distinct()