mongo.scala

import net.liftweb.mongodb.record.field.ObjectIdField
import net.liftweb.mongodb.record.{MongoMetaRecord, MongoRecord}
import net.liftweb.record.field.IntField

// ==== BASE

trait BaseDoc[T <: BaseDoc[T]] extends MongoRecord[T] {

  self: T =>

  def id = _id.value

  object _id extends ObjectIdField(this.asInstanceOf[T])
  object int extends IntField(this.asInstanceOf[T])
}

trait BaseMeta[T <: BaseDoc[T]] extends MongoMetaRecord[T] {

  self: T =>
}

// ==== DOC #1

class BaseDoc1 extends BaseDoc[BaseDoc1] {
  def meta = BaseMeta1
}
object BaseMeta1 extends BaseDoc1 with BaseMeta[BaseDoc1]

// ==== DOC #2

class BaseDoc2 extends BaseDoc[BaseDoc2] {
  def meta = BaseMeta2
}
object BaseMeta2 extends BaseDoc2 with BaseMeta[BaseDoc2]

// ==== FACTORY

case class Mod(num: Int) {

  def doc: BaseMeta[_] = // WRONG 
    num % 2 match {
      case 0 => BaseMeta1
      case _ => BaseMeta2
    }
}

object Main extends App {
  val doc = Mod(0).doc.createRecord.int(5)
}

the-error

error: value int is not a member of _$1
val doc = Mod(0).doc.createRecord.int(5)