Scala Cookbook

16.7. Accessing the MongoDB Document ID Field

Problem

You want to get the ID field for a document you’ve inserted into a MongoDB collection.

Solution

Perform a query to get the document you want, and then call get("_ID") on the resulting MongoDBObject, like this:

basicDbObject.get("_id")

The following example shows how to get the ID field from a DBObject after inserting the object into the database. I first create a Stock as usual, convert the Stock to a MongoDBObject, perform the insert, and then get the ID value, which is added to the MongoDBObject after the insert operation is performed:

import com.mongodb.casbah.Imports._
import Common._

object InsertAndGetId extends App {

  val coll = MongoFactory.collection

  // get the _id field after an insert
  val amazon = Stock("AMZN", 220)
  val amazonMongoObject = buildMongoDbObject(amazon)
  coll.insert(amazonMongoObject)
  println("ID: " + amazonMongoObject.get("_id"))

}

If you just need to get the ID field from a MongoDBObject after performing a query, the following complete example shows how to do that with a match expression:

import com.mongodb.casbah.Imports._

object GetId extends App {

  val collection = MongoFactory.collection

  val query = MongoDBObject("symbol" -> "GOOG")
  collection.findOne(query) match {
    case Some(result) => println("ID: " + result.get("_id"))
    case None => println("Stock not found")
  }

}

A match expression is used in this example because the findOne(query) will return None if no matching documents are found in the collection. You ...