Feature-request: Document.save(on_conflict="replace")

I have a feature-request to optionally replace the document on DuplicateKeyError. Those who are familiar with postgres, might know the ON CONFLICT option in insert-statements. This would be similar.

The part to extend is this one in document.py:

except pymongo.errors.DuplicateKeyError as err: message = "Tried to save duplicate unique keys (%s)" raise NotUniqueError(message % err)

So if on_conflict="replace" then the document in db would be deleted and the new one inserted.

Further on_conflict strategies could be:

• "update": get document from db, update it with new document
• "ignore" : do not throw an exception
• None: raise exception as it already does.

with default on_conflict=None it should be backwards-compatible

Start in document.py at the DuplicateKeyError handling in the save path and inspect how documents are inserted. Confirm which conflict strategy is in scope, with replace explicitly requested and update, ignore, or None proposed; done means the selected behavior works while the default None remains backward-compatible.