Doing a MongoDB findAndModify query including $addToSet in Casbah

I am both new to scala and cashbah. I am trying to

update a document if exists (by _id) and create if doesn't exist.
while updating, update some key values
while updating, update some keys which values are sets, include some data to those sets.

To achive this, I've written this:

DBObject = MongoDBObject("_id" -> uri.toString) ++
      $addToSet("appearsOn" -> sourceToAppend) ++
      $addToSet("hasElements" -> elementsToAppend) ++
      $addToSet("hasTriples" -> triplesToAppend) ++
      MongoDBObject("uDate" -> new DateTime)

    /* Find and replace here! */
    OntologyDocument.dao.collection.findAndModify(
      query = MongoDBObject({"_id" -> uri.toString}),
      update = update,
      upsert = true,
      fields = null,
      sort = null,
      remove = false,
      returnNew = true
    )

Documents looked by _id, some new items added to appearsOn hasElements hasTriples and uDate is updated.

sourceToAppend elementsToAppend and triplesToAppend are List[String]

When I run this, I got this error:

java.lang.IllegalArgumentException: fields stored in the db can't start with '$' (Bad Key: '$addToSet')
    at com.mongodb.DBCollection.validateKey(DBCollection.java:1444) ~[mongo-java-driver-2.11.1.jar:na]

I didn't get it. What is wrong with this query? $addToSet isn't a field, why casbah thinks it is a field? What am I doing wrong here?

Solution

The reason its failing is because the update query is invalid (it wont work in the js shell).

$set is implicit for values in the update document, but you can't mix it with other update operators eg $addToSet. If you want to mix $set with other set operators then you can if you are explicit:

val update = $set("uDate" -> new DateTime) ++ 
             $addToSet("appearsOn" -> sourceToAppend, 
                       "hasElements" -> elementsToAppend, 
                       "hasTriples" -> triplesToAppend)

You can't $set "_id" but as thats in the query and its an upsert - it will merge so don't include it in the update statement - otherwise it will error.

Finally, @AsyaKamsky is right if you dont need the returned document - use an update its also atomic.