I suggest you make a Draft schema with a separate DB table with one column: metadata, of type :map (jsonb in Postgres). Save drafts there as a free-form Elixir map and load them back later for further refinement.

I think that bypassing validation is always a bad idea.
Add draft_changeset function and use it for the draft purpose, it will avoid your database to be full of strange things

If you are asking how to cast_assoc for a specific changeset, that can be specified via the with option… Ecto.Changeset — Ecto v3.14.0

def draft_changeset(article, params) do
  article
  |> cast(params, [:title, :published_at])
  |> cast_assoc(:contents, with: &MyBlog.Content.draft_changeset/2)
end

This might be enough. Depending on what you’re doing @dimitarvp 's draft suggestion or a separate service module with an Ecto multi and customized validation and persistence across multiple entities might make sense.