How to update on doctrine’s persist method by custom field

Good morning,

normally on $em->persist($variable) doctrine checks the id field if the entry exist. In my case, i get the entries to insert/update from a file where a id, named òld_id by example. This id is stored in my Database.

Now i try to update the db entries where doctrine should compare the old_id field from database and file.

Is this possible?

I could fetch all database entries before persist() and compare myself. But if is there a better solution, maybe to set a custom id field i would try this.