For anyone coming here via Google, something like this:

alter table("comments") do
  modify :post_id, references(:posts, on_delete: :delete_all),
    from: references(:posts, on_delete: :nothing)
end

Now works as per example here in docs. Not sure exactly when this changed but hopefully saves someone some time. Tested with Ecto 3.10.3

yea so you would probably want to drop the fk. you will need to set up/down however because ecto wont know how to rollback. So

def up do
  drop constraint(:foo, "foo_bar_id_fkey")

  alter table(:foo) do
    modify :bar_id, references(:bar, on_delete: :delete_all)
  end
end

def down do
  drop constraint(:foo, "foo_bar_id_fkey")

  alter table(:foo) do
    modify :bar_id, references(:bar, on_delete: :nothing)
  end
end

I am surprised this does not work (unless it does?).
This would save you writing up and down

def change do
  drop constraint(:foo, "foo_bar_id_fkey")

  alter table(:foo) do
    modify :bar_id, references(:bar, on_delete: :delete_all), from: references(:bar, on_delete: :nothing)
  end
end