How to temporarily violate a unique constraint?

[MarcelloTheArcane]

I have the following table:

id	title	chapter
1	The Cat in The Hat	1
2	The Cat in The Hat	2
3	The Cat in The Hat	3
4	War and Peace	1
5	War and Peace	2

I have a unique constraint ( title, chapter ), to make sure that you can't have duplicate chapters in a given book.

How can I swap two chapters? If I try to do it with a mutation

mutation swap ($from_id, $from_chapter, $to_id, $to_chapter) {
  a: update_books_by_pk (
    pk_columns: {
      id: $from_id
    }
    _set: {
      chapter: $to_chapter
    }
  ) {
    id
    title
    chapter
  }

  b: update_books_by_pk (
    pk_columns: {
      id: $to_id
    }
    _set: {
      chapter: $from_chapter
    }
  ) {
    id
    title
    chapter
  }
}

I get a Uniqueness violation. duplicate key value violates unique constraint books_title_chapter_index error.

I have thought about updating in two stages, for example by updating one chapter to a known-impossible value (like -1) and then updating again to the swapped value, but this creates a race condition and leaves the database in an incorrect state if the second request fails for some reason.

Is it possible to have a post-update trigger that enforces the constraint? How would that react if someone tried an incorrect update?