private Entity findById(String id) {
    return repository.findById(id).fetchFirst().getUnchecked().get();
  }
When upsert is requested on different versions but same ID there should be duplicate
key exception thrown by Mongo since there will be an attempt to insert new document (same id
different version)
Based on criteria it is a new document, based on primary key ({@code _id}) it exists already.
mongo/test/org/immutables/mongo/fixture/SimpleReplacerTest.java