This should make only one DB query.
post_query =
from p in Post,
where: p.id == ^id,
join: c in assoc(p, :comments),
join: u in assoc(c, :user),
preload: [comments: {c, user: u}]
Regarding selecting association fields, you can do something like this (not tested)
post_query =
from p in Post,
where: p.id == ^id,
join: c in assoc(p, :comments),
join: u in assoc(c, :user),
select: %{id: p.id, title: p.title, comments: p.comments, name: u.name}
Not sure exactly what you want to get back but that’s one way to select fields 
