title: MongoDB: Insert a document if not exist, but skip updating in case it already exists
body: <p>How do I insert a document in mongoose if not exist without doing an update on the existing one if it already exists? I know if I do an upsert it tries to update if exist and inserts a new document if it doesn't exist. However, my situation is that I just want to insert a new document if not exist and leave the existing one without updating it.</p>