You sure you are getting this error from this query? Because I saw this error when we are calling some nested schema attribute from views and its association is not preloaded.
On side note you can optimise your query:
(from p in Post,
preload: [:author, [shared: :author]],
where: p.inserted_at < ^stamp and p.author_id == ^author),
|> order_by(desc: :inserted_at)
|> Repo.all()
