Django queryset to return first of each item in foreign key based on date

Question

need to get a queryset with the first book (by a date field) for each author (related to by foreign key) ...is there a Django ORM way to do this (without custom SQL preferre

Answer 1

If you use PostgreSQL or another DB backend with support for DISTINCT ON there is a nice solution:

Books.objects.order_by('author', '-date').distinct('author')

Otherwise I don't know a solution with only one query. But you can try this:

from django.db.models import Q, Max
import operator

books = Book.objects.values('author_id').annotate(max_date=Max('date'))
filters = reduce(operator.or_, [(Q(author_id=b['author_id']) &
    Q(date=b['max_date'])) for b in books])
queryset = Books.objects.filter(filters)

With the combination of .values() and .annotate() we group by the author and annotate the latest date of all books from that author. But the result is a dict and not a queryset. Then we build a SQL statement like WHERE author_id=X1 AND date=Y1 OR author_id=X2 AND date=Y2.... Now the second query is easy.