how do I make a reversible reference migration, i.e. adding the referential constraint on “up” action and dropping it on “down” action, using from: keyword?
I’ve a pretty big migration and it’s much easier to use “def change” than “def up” and “def down”.

The documentation for references/3 says it can be used like such:

# Modify the :on_delete option of an existing foreign key
alter table("comments") do
  modify :post_id, references(:posts, on_delete: :delete_all),
    from: references(:posts, on_delete: :nothing)
end

which, I assume, would drop and recreate the constraint and is safe to use in “change” mode. How do I write one where “from” is a “no FK constraint” state?