A simple example showing two ways of getting IDs of documents inserted using ReactiveMongo.

Main.scala

import scala.concurrent.ExecutionContext.Implicits.global
import reactivemongo.bson.BSONObjectID
import reactivemongo.bson.BSONDocument
import scala.concurrent.Future

object Main {

  def main(args: Array[String]): Unit = {
    val driver = new reactivemongo.api.MongoDriver
    val connection = driver.connection(List("localhost"))
    val dbFuture = connection.database("mongotest")

    val collectionFuture = dbFuture.map(db => db.collection("foo"))

    // Using for to run futures sequentially
    val queriesFuture = for {
      collection    <- collectionFuture
      removeResult  <- collection.remove(BSONDocument())
      insert0Result <- collection.insert(BSONDocument("bar" -> 0))
      id1           <- Future { BSONObjectID.generate() }
      insert1Result <- collection.insert(BSONDocument("_id" -> id1, "bar" -> 1))
      insert2Result <- collection.findAndUpdate(
        selector       = BSONDocument("bar" -> ""), // Make sure this won't match any document!
        update         = BSONDocument("bar" -> 2),
        fetchNewObject = true,
        upsert         = true)
      findResult    <- collection.find(BSONDocument()).cursor[BSONDocument].collect[List]()
    } yield {
      println("id0: ?")
      println("id1: " + id1)
      insert2Result.value.map(doc => println("id2: " + doc.getAs[BSONObjectID]("_id").get))
      println("All documents:")
      findResult.map(doc => println(BSONDocument.pretty(doc)))
      println("Done.")
    }

    queriesFuture.onComplete(_ => driver.close())
  }

}

Sample output:

id0: ?
id1: BSONObjectID("582e452d0701000701808ac5")
id2: BSONObjectID("582e452dc3e2f7fa19d7413c")
All documents:
{
  _id: BSONObjectID("582e452dc3e2f7fa19d7413b"),
  bar: BSONInteger(0)
}
{
  _id: BSONObjectID("582e452d0701000701808ac5"),
  bar: BSONInteger(1)
}
{
  _id: BSONObjectID("582e452dc3e2f7fa19d7413c"),
  bar: BSONInteger(2)
}
Done.