Question

class Book(models.Model):
    name = models.CharField(max_length=127, blank=False)

class Author(models.Model):
    name = models.CharField(max_length=127, blank=False)
    books = models.ManyToMany(Books)

I am trying to filter the authors so I can return a result set of authors like:

[{id: 1, name: 'Grisham', books : [{name: 'The Client'},{name: 'The Street Lawyer}], ..]

Before I had the m2m relationship on author I was able to query for any number of author records and get all of the values I needed using the values method with only one db query.

But it looks like

Author.objects.all().values('name', 'books')

would return something like:

[{id: 1, name: 'Grisham', books :{name: 'The Client'}},{id: 1, name: 'Grisham', books :{name: 'The Street Lawyer'}}]

Looking at the docs it doesn’t look like that is possible with the values method.
https://docs.djangoproject.com/en/dev/ref/models/querysets/

Warning Because ManyToManyField attributes and reverse relations can
have multiple related rows, including these can have a multiplier
effect on the size of your result set. This will be especially
pronounced if you include multiple such fields in your values() query,
in which case all possible combinations will be returned.

I want to try to get a result set of n size with with the least amount of database hits authorObject.books.all() would result in at least n db hits.

Is there a way to do this in django?

I think one way of doing this with the least amount of database hits would be to :

authors = Authors.objects.all().values('id')
q = Q()
for id in authors:
   q = q | Q(author__id = id)

#m2m author book table.. from my understanding it is 
#not accessible in the django QuerySet
author_author_books.filter(q)  #grab all of the book ids and author ids with one query

Is there a built in way to query the m2m author_author_books table or am I going to have the write the sql? Is there a way to take advantage of the Q() for doing OR logic in raw sql?

Thanks in advance.

Answer 1

I think you want prefetch_related. Something like this:

authors = Author.objects.prefetch_related('books').all()

More on this here.

If you want to query your author_author_books table, I think you need to specify a “through” table:

class BookAuthor(models.Model):
  book = models.ForeignKey(Book)
  author = models.ForeignKey(Author)

class Author(models.Model):
  name = models.CharField(max_length=127, blank=False)
  books = models.ManyToMany(Books, through=BookAuthor)

and then you can query BookAuthor like any other model.