Mongodb scala driver codec for trait and inherited classes

Question

Using the following mongo-driver . I want to insert and get structures (see below) from MongoDB.

 trait A {
   def id: ObjectId
 }

 case class B(id: ObjectId) extends A

 case class C(id: ObjectId, name: String) extends A

I find a solution with using sealed classes, but I want to use traits. I want to find a solution with Codecs or something else.

Answer 1

I had the same concern just a few days ago but didn't find anything in the documentation regarding sealed traits for modeling ADT in MongoDB.

In the end, I used sealed class as suggested in the official scala driver github repo .

If you really want to use traits (due to the definition of abstract methods) you can do something like this:

package example.model

import example.model.adt._
import org.mongodb.scala.bson.ObjectId
import org.mongodb.scala.bson.codecs.Macros._
import org.mongodb.scala.bson.codecs.DEFAULT_CODEC_REGISTRY
import org.bson.codecs.configuration.CodecRegistries.{fromProviders, fromRegistries}

trait MongoModel {
  def _id: ObjectId
}

object MongoModel {
  val codecRegistery = fromRegisteries(fromProviders(classOf[A]), DEFAULT_CODEC_REGISTRY)
}

Now you can have your ADT for A defined with sealed class .

package example.model.adt

import example.model.MongoModel
import org.mongodb.scala.bson.ObjectId

sealed class A

final case class B(_id: ObjectId) extends A with MongoModel
final case class C(_id: ObjectId) extends A with MongoModel

This answer doesn't solve the question directly but provides a feasible workaround. Note that this code is just an example. For a more complete implementation, you can see this github repo .

Answer 2

Since release 2.7, the mongodriver is now able to serialize sealed traits .
It works exactly like serializing a sealed classes.