[{"collection":"collection","count":10,"content":{"_id":{"type":"autoincrement","autoType":"int","startInt":0},"uniqueField":{"type":"string","minLength":5,"maxLength":5}}}]

db.collection.update({},[{"$set":{"uniqueField":{"$concat":[{"$toString":"$_id"},"_","$uniqueField"]}}}],{"multi":true,"upsert":false})