Getting a collection

An instance of MongoCollection[F, T] can be obtained from an existing database instance by specifying collection’s name:

import mongo4cats.bson.Document
import mongo4cats.collection.MongoCollection

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

Alternatively, if collection needs to be tied to a specific class, MongoDatabase[F] has special methods for doing this as well:

// needs to have an instance of CodecRegistry built for the provided class
val collection: IO[MongoCollection[IO, MyClass]] = database.getCollection[MyClass]("mycoll", myClassCodecRegistry)

// needs to have an instance of MongoCodecProvider[MyClass] available in the implicit scope
val collection: IO[MongoCollection[IO, MyClass]] = database.getCollectionWithCodec[MyClass]("mycoll")

More information on MongoDB codecs and codec registries can be found in the official documentation. One of the supported options for deriving MongoDB codecs is through the use of the popular Json library for Scala - Circe.

If a collection that you are trying to obtain does not exist, it will be created by MongoDB during the first query. Additionally, MongoDatabase[F] has methods for creating collections explicitly:

val collection: IO[Unit] = database.createCollection("mycoll")

// or with options
import mongo4cats.collection.CreateCollectionOptions

val options = CreateCollectionOptions().capped(true).sizeInBytes(1024L)
val collection: IO[Unit] = database.createCollection("mycoll", options)