Question

I am looking to solve this optimally using Django Query Set.

Each library can have one and only one book.
Several libraries can have multiple instances of the same book (i.e.: Library1 can have an instance of “Girl with the Dragon Tattoo” and Library4 can have another instance of “Girl with the Dragon Tattoo”)

Book(models.Model):
    name = models.CharField(max_length=30)

Library(models.Model):
    book = models.ForeignKey(Book)    # multiple libraries have the book
    city = models.CharField(max_length=50)

So how would I find a book that is in less than 3 libraries?

Answer 1

Use annotate and Count.

 Book.objects.annotate(n_libraries=Count('library')).filter(n_libraries__lt=3)