For a walkthrough of the main CRUD operations please refer to the Quick Tour.

All CRUD-related methods in the Scala driver are accessed through the MongoCollection case class. Instances of MongoCollection can be obtained from a MongoClient instance by way of a MongoDatabase:

val client: MongoClient = MongoClient()
val database: MongoDatabase = client.getDatabase("mydb")
val collection: MongoCollection[Document] = database.getCollection("mycoll")

MongoCollection takes the type of TDocument which defines the
class that clients use to insert or replace documents in a collection, and the default type returned from find and aggregate.

The single-argument getCollection method returns an instance of MongoCollection[Document], and so with this type of collection an application uses instances of the immutable Document class:

val collection: MongoCollection[Document] = database.getCollection("mycoll")

// insert a document
val document: Document = Document("_id" -> 1, "x" -> 1)
val insertObservable: Observable[Completed] = collection.insertOne(document)

insertObservable.subscribe(new Observer[Completed] {
  override def onNext(result: Completed): Unit = println(s"onNext: $result")
  override def onError(e: Throwable): Unit = println(s"onError: $e")
  override def onComplete(): Unit = println("onComplete")


val replacementDoc: Document = Document("_id" -> 1, "x" -> 2, "y" -> 3)

// replace a document
collection.replaceOne(Filters.eq("_id", 1), replacementDoc
    ).subscribe((updateResult: UpdateResult) => println(updateResult))


// find documents
collection.find().collect().subscribe((results: Seq[Document]) => println(s"Found: #${results.size}"))


See the Observables documentation for more information about Observables and implicit helpers.


An overload of the getCollection method allows clients to specify a different class for representing BSON documents. For example, users my wish their own class with the CRUD API directly. Below we use the BsonDocument class from the Scala driver directly:

// Pass BsonDocument.class as the second argument
val collection: MongoCollection[BsonDocument] = database.getCollection("mycoll", BsonDocument.class)

// insert a document
val document: BsonDocument = new BsonDocument("_id", new BsonInt32(2)).append("x", new BsonInt32(1))
collection.insertOne(document).subscribe((x: Completed) => println("Inserted"))


val replacementDoc: BsonDocument = new BsonDocument("_id", new BsonInt32(2)).append("x", new BsonInt32(2)).append("y", new BsonInt32(3))

// replace a document
collection.replaceOne(Filters.eq("_id", document.getInt32("1")), replacementDoc).subscribe((updateResult: UpdateResult) => println(updateResult))


// find documents
collection.find().collect().subscribe((results: Seq[BsonDocument]) => println(s"Found BsonDocuments: #${results.size}"))

There are two requirements that must be met for any class used in this way:

  • a Codec for it must be registered in the MongoCollection’s CodecRegistry
  • the Codec must be one that encodes and decodes a full BSON document (and not just, for example, a single BSON value like an Int32)

By default, a MongoCollection is configured with Codecs for two classes:

  • Document
  • BsonDocument

Applications, however, are free to register Codec implementations for other classes by customizing the CodecRegistry. New CodecRegistry instances are configurable at three levels:

  • In a MongoClient via MongoClientOptions
  • In a MongoDatabase via its withCodecRegistry method
  • In a MongoCollection via its withCodecRegistry method

Consider the case of encoding and decoding instances of the UUID class. The Scala driver by default encodes instances of UUID using a byte ordering that is not compatible with other MongoDB drivers, and changing the default would be quite dangerous. But it is possible for new applications that require interoperability across multiple drivers to be able to change that default, and they can do that with a CodecRegistry.

// Replaces the default UuidCodec with one that uses the new standard UUID representation
val codecRegistry: CodecRegistry = 
CodecRegistries.fromRegistries(CodecRegistries.fromCodecs(new UuidCodec(UuidRepresentation.STANDARD)),

// globally
val clientSettings: MongoClientSettings = MongoClients.create("mongodb://localhost").getSettings()
newClientSettings = MongoClientSettings.builder(clientSettings).codecRegistry(codecRegistry).build()
val client: MongoClient = MongoClient(newClientSettings)

// or per database
val database: MongoDatabase = client.getDatabase("mydb")

// or per collection
val collection: MongoCollection<Document> = database.getCollection("mycoll")

Write Concern

Applications can configure the WriteConcern that a MongoCollection uses for write operations. Like CodecRegistry, the WriteConcern can be configured at three levels:

  • In a MongoClient via MongoClientOptions
  • In a MongoDatabase via its withWriteConcern method
  • In a MongoCollection via its withWriteConcern method

Read Preference

Applications can configure the ReadPreference that a MongoCollection uses for read operations. Like WriteConcern, the ReadPreference can be configured at three levels:

  • In a MongoClient via MongoClientOptions
  • In a MongoDatabase via its withReadPreference method
  • In a MongoCollection via its withReadPreference method

Immutability of MongoDatabase and MongoCollection

Instance of MongoDatabase and MongoCollection are immutable, so rather than mutate the state of the MongoCollection on which they are invoked, the three methods discussed above return new instances. Applications should therefore be sure to store the result of the method call. For example:

// CORRECT: The results of the method calls are chained and the final one is referenced 
// by collection 
val collection: MongoCollection[Document] = database.getCollection("mycoll")

// INCORRECT: withReadPreference returns a new instance of MongoCollection
// It does not modify the collection it's called on.  So this will
// have no effect