Group by limit 1 in django

Question

I have the following models in Django (simplified for brevity):

class DistinctWord(models.Model):
    ...

class Word(models.Model):
    distinct_word = models.F         


        

Answer 1

You can do this using the Subquery API:

from django.db.models.expressions import Subquery, OuterRef

first_word = Word.objects.filter(
    distinct_word=OuterRef('distinct_word')
).order_by('pk').values('pk')[:1]

UserWord.objects.filter(
     # whatever filters...
).annotate(
     first_word=Subquery(first_word)
)

This will result in SQL that looks something like:

SELECT user_word.*,
       (SELECT word.id 
          FROM word 
         WHERE word.distinct_word_id = user_word.distinct_word_id
       ) AS first_word
  FROM user_word
 WHERE ...

This will probably not perform as well as a JOIN with a DISTINCT ON in postgres, and may not perform as well as a JOIN with a GROUP BY, as it will need to execute the subquery for each row.