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Django ORM Optimization Tips #3 aggregation

shawara profile image Mahmoud Shawara Updated on ・3 min read

Today we are going to discuss GROUP BY with aggregation(SUM)

Suppose we have Tables of users , playlists and songs.

  • Each user can have one or more playlists.
  • Each playlist can have one or more songs
# models
class User(models.Model):
    name = models.CharField(max_length=127)

    @property
    def playlists_total_length(self) -> int:
        return sum([p.songs_total_length for p in self.playlists.all()])


class Playlist(models.Model):
    user = models.ForeignKey(User, related_name='playlists')
    name = models.CharField(max_length=255)

    @property
    def songs_total_length(self) -> int:
        return sum([song.length for song in self.songs.all()])


class Song(models.Model):
    playlist = models.ForeignKey(Playlist, related_name='songs')
    title = models.CharField(max_length=255)
    length = models.PositiveIntegerField(help_text=_('In seconds'))

So if our goal is to list all users with playlists_total_length

How bad is this code?

If we had for example:

  • 10 Users
  • 10 Playlists per user
  • 10 songs per playlist

Total objects to fetch = 10 (Users) + 10*10 (Playlists) + 10*10*10 (Songs)

1110 DB rows in 3 different tables for just 10 users

Solution: GROUP BY with aggregation(SUM)

  • Use aggregation to calculate total length of all songs: Song.objects.aggregate(Sum('length'))
SELECT SUM("song"."length") AS "length__sum" FROM "song";

Result: {'length__sum': 490}

  • Use annotate for Group by: Song.objects.annotate(Sum('length'))
SELECT * , SUM("song"."length") AS "length__sum" 
FROM "song" 
GROUP BY "song"."id", 
         "song"."playlist_id", 
         "song"."title",
         "song"."length

Result: <QuerySet [Song(...),Song(...),...]>

  • Use values before annotate to group by specific columns: Song.objects.values('playlist_id').annotate(Sum('length'))
SELECT "song"."playlist_id", SUM("song"."length") AS "length__sum"
FROM "song" 
GROUP BY "song"."playlist_id"

Result: <QuerySet [{'playlist_id': 2, 'length__sum': 310}, ...]>

Song.objects.values('playlist_id').annotate(Sum('length')).values_list('length__sum')

We can compress this query and still getting the same generated sql query:

Song.objects.values('playlist_id').values_list(Sum('length'))

SELECT SUM("song"."length") AS "length__sum" 
FROM "song" 
GROUP BY "song"."playlist_id"

Result: <QuerySet [(310,), (180,)]>

List all Playlists with total songs length:

songs_subquery = Subquery(
    queryset=Song.objects
        .values('playlist_id') \
        .filter(playlist__id=OuterRef('id')) \
        .values_list(Sum('length')),
    output_field=IntegerField())

queryset = Playlist.objects.annotate(
    _songs_total_length=songs_subquery)

OuterRef will make it possible to access fields from parent query when you are in subquery. OuterRef('id') -> "playlist"."id"

SELECT "playlist"."id",
       "playlist"."user_id",
       "playlist"."name",
       (
           SELECT SUM("song"."length")
               FROM "song"
               WHERE "song"."playlist_id" = ("playlist"."id")
               GROUP BY "song"."playlist_id" LIMIT 1
        ) AS "songs_total_length"
    FROM "playlist";

List all Users with total songs length:

songs_subquery = Subquery(
    queryset=Song.objects
        .values('playlist__id') \
        .filter(playlist__id=OuterRef('id')) \
        .values_list(Sum('length')),
    output_field=IntegerField(default=0))

playlists_subquery = Subquery(
    queryset=Playlist.objects
        .values('user__id') \
        .filter(user__id=OuterRef('id')) \
        .annotate(_songs_total_length=Sum(songs_subquery)) \
        .values_list('_songs_total_length'),
    output_field=IntegerField(default=0))

queryset = User.objects.annotate(_playlists_total_length=playlists_subquery)
SELECT "user"."id",
       "user"."name",
       (SELECT SUM((SELECT SUM(U0."length") AS "length__sum"
                    FROM "song" U0
                    WHERE U0."playlist_id" = V0."id"
                    GROUP BY U0."playlist_id")) AS "_songs_total_length"
        FROM "playlist" V0
        WHERE V0."user_id" = "user"."id"
        GROUP BY V0."user_id") AS "_playlists_total_length"
FROM "user";

It will be so hard to write this complex query in everywhere you want

So here is the final clean code:

class UserQuerySet(models.QuerySet):
    @classmethod
    def playlists_total_length(cls):
        playlist_annotation = PlaylistQuerySet.songs_total_length()

        queryset = Playlist.objects \
            .values('user__id') \
            .filter(user__id=OuterRef('id')) \
            .values_list(Sum(playlist_annotation))

        return Subquery(
            queryset=queryset,
            output_field=IntegerField()
        )

    def collect(self):
        return self.annotate(_playlists_total_length=self.playlists_total_length())


class User(models.Model):
    objects = UserQuerySet.as_manager()
    name = models.CharField(max_length=255)

    @property
    def playlists_total_length(self):
        if hasattr(self, '_playlists_total_length'):
            return self._playlists_total_length
        return sum([p.songs_total_length for p in self.playlists.all()])



class PlaylistQuerySet(models.QuerySet):
    @classmethod
    def songs_total_length(cls):
        queryset = Song.objects \
            .values('playlist__id') \
            .filter(playlist__id=OuterRef('id')) \
            .values_list(Sum('length'))  # The SUM aggregation

        return Subquery(
            queryset=queryset,
            output_field=models.IntegerField()
        )

    def collect(self):
        return self.annotate(_songs_total_length=self.songs_total_length())


class Playlist(models.Model):
    objects = PlaylistQuerySet.as_manager()
    user = models.ForeignKey(User, related_name='playlists')
    name = models.CharField(max_length=255)

    @property
    def songs_total_length(self):
        if hasattr(self, '_songs_total_length'):
            return self._songs_total_length
        return sum([song.length for song in self.songs.all()])



class Song(models.Model):
    playlist = models.ForeignKey(Playlist, related_name='songs')
    title = models.CharField(max_length=255)
    length = models.PositiveIntegerField(help_text='In seconds')

Thanks, Ivo Donchev

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