How to query with '$in' over '_id' in reactive mongo and play

Question

I have a project set up with playframework 2.2.0 and play2-reactivemongo 0.10.0-SNAPSHOT. I'd like to query for few documents by their ids, in a fashion similar to this:

def usersCollection = db.collection[JSONCollection]("users")
val ids: List[String] = /* fetched from somewhere else */
val query = ??
val users = usersCollection.find(query).cursor[User].collect[List]()

As a query I tried:

Json.obj("_id" -> Json.obj("$in" -> ids))                        // 1
Json.obj("_id.$oid" -> Json.obj("$in" -> ids))                   // 2
Json.obj("_id" -> Json.obj("$oid" -> Json.obj("$in" -> ids)))    // 3

for which first and second return empty lists and the third fails with error assertion 10068 invalid operator: $oid.

Answer 1

NOTE: copy of my response on the ReactiveMongo mailing list.

First, sorry for the delay of my answer, I may have missed your question. Play-ReactiveMongo cannot guess on its own that the values of a Json array are ObjectIds. That's why you have to make a Json object for each id that looks like this: {"$oid": "526fda0f9205b10c00c82e34"}. When the ReactiveMongo Play plugin sees an object which first field is $oid, it treats it as an ObjectId so that the driver can send the right type for this value (BSONObjectID in this case.)

This is a more general problem actually: the JSON format does not match exactly the BSON one. That's the case for numeric types (BSONInteger, BSONLong, BSONDouble), BSONRegex, BSONDateTime, and BSONObjectID. You may find more detailed information in the MongoDB documentation: http://docs.mongodb.org/manual/reference/mongodb-extended-json/ .