Reactive Scala Driver for MongoDB

Asynchronous & Non-Blocking

Collection API

The Collection API is designed to be very extensible, allowing the use of third-party libraries for building documents (e.g. use something else than the embedded BSON library), including specific serializers and deserializers.

For example, that’s the case of the support of Play JSON, which relies on the Play JSON library instead.

// using the default Collection implementation
// (relying on the embedded BSON library)

import scala.concurrent.Future

import reactivemongo.bson.{ BSONDocument, BSONDocumentReader, Macros }
import reactivemongo.api.collections.bson.BSONCollection

case class Person(name: String, age: Int)

object Person {
  implicit val reader: BSONDocumentReader[Person] = Macros.reader[Person]

def db1: reactivemongo.api.DefaultDB = ???

val collection1: BSONCollection = db1.collection("people")

// find people who are older than 25
val query1 =
  BSONDocument("age" -> BSONDocument("$gt" -> 25))

// run the query then convert the result to a `Person` instance
// using the implicit (BSON) reader
val result1: Future[Option[Person]] = collection1.find(query1).one[Person]
// using the Play plugin's Collection implementation
// (relying on Play's JSON library)
import scala.concurrent.Future

import play.api.libs.json._
import play.modules.reactivemongo.json.collection.JSONCollection

def db2: reactivemongo.api.DefaultDB = ???

val collection2 = db2.collection[JSONCollection]("people")

// find people who are older than 25
val query2 = Json.obj("age" -> Json.obj("$gt" -> 25))

// run the query then convert the result to a `Person` instance
// using the implicit (JSON) reader
def result2(implicit jsonReads: Reads[Person]): Future[Option[Person]] =

This is very useful when you don’t want to explicitly convert your objects into yet another different structure – if your application uses JSON, it is perfectly understandable to want to avoid using BSON only for dealing with MongoDB.

The Collection trait

This trait is almost empty.

package simplified.api

import reactivemongo.api.DB

// simplified for the sake of readability
trait Collection {
  /** The database which this collection belong to. */
  def db: DB

  /** The name of the collection. */
  def name: String

  /** Gets the full qualified name of this collection. */
  def fullCollectionName = + "." + name

All collections implementations must mix this trait in. They also provide implicit objects of type CollectionProducer that make new (specialized) instances of them. Since db.collection() is parametrized with C <: Collection and accepts an implicit CollectionProducer[C], the returned instance of collection can be inferred to the right type if there is only one producer in the implicit scope, which is a typical situation.

package simplifiedapi

import reactivemongo.api.{ Collection, CollectionProducer }

trait DB {
  def collection[C <: Collection](name: String)(implicit producer: CollectionProducer[C])

Most implementations actually extend another trait, GenericCollection.

The GenericCollection trait

This trait is much more complete than Collection. It defines common methods, like find(), update(), remove() and insert(), among others. One particularity of them is that they may be given …

GenericCollection takes 1 type parameter: the SerializationPack.


Let’s take an example of how these types are used with find(), which is defined like this:

package simplifiedapi

import reactivemongo.api.collections.GenericQueryBuilder

trait GenericCollection {
  def serializationPack: reactivemongo.api.SerializationPack
  val pack = serializationPack

  def find[S](selector: S)(implicit swriter: pack.Writer[S]): GenericQueryBuilder[pack.type]

This method takes a selector (or query), of type S. This object is then transformed into BSON thanks to the implicit swriter parameter. Moreover, you can notice that the return type is another trait, GenericQueryBuilder, with the same parameter types.

The GenericQueryBuilder trait

A GenericQueryBuilder, like its name says it, helps building and customizing the query.