Django: add additional data in queryset

Question

I've two models:

Movie

Attributes : name, plot, rating, release_date, photo

Director

Attributes : name, bio, photo

Movie and Director models has a many-to-many relationship.

Now in my movies/views.py file:

def movies_index(request):
    movies = Movie.objects.all()

    for movie in movies:
       movie.directors = movie.directors.all() # tried but in the template it gives me error when accessing by movie.directors   
       # insert directors in every movie object 

    context_data = {
       'movies': movies
    }

    return render(request, 'movies/index.html', context_data)

I want to insert directors data in to every movie object. So that the result may be like:

[
  {
    name: "12 Angry Man",
    plot: "bla",
    release_date: "1-2132",
    directors: [
      {
         name: "Quentine",
         bio: "Lodfs"
      },
      {
         name: "Tarantino",
         bio: "Lodsdfs"
      }
    ]
  }
]

How can I achieve this?

Or, is there any different approach do do this?

NB I'm using Django 1.9, Python 2.7

Answer 1

Yes, there is a different approach that does not involve looping and executing N more queries to fetch the related director objects.

prefetch_related

has a similar purpose to select_related, in that both are designed to stop the deluge of database queries that is caused by accessing related objects, but the strategy is quite different.

select_related works by creating an SQL join and including the fields of the related object in the SELECT statement. For this reason, select_related gets the related objects in the same database query. However, to avoid the much larger result set that would result from joining across a 'many' relationship, select_related is limited to single-valued relationships - foreign key and one-to-one.

prefetch_related, on the other hand, does a separate lookup for each relationship, and does the 'joining' in Python. This allows it to prefetch many-to-many and many-to-one objects, which cannot be done using select_related,

So

 Movie.objects.all().prefetch_related('director')