Ecto Query - aggr many-to-many count child

ecto_query.exs

# user <1---*> playlist <1---*> playlist_song
(from p in Playlist,
    join: u in assoc(p, :user),
    left_join: ps in assoc(p, :songs),
    where: p.id == ^1 and u.id == ^1,
    group_by: p.id,
    select: %{
        playlist_id: p.id,
        song_count: count(ps.id)
    }
) |> Repo.one()