object Holder {
    var companions = Set[AnyRef]()

    def companionFor[M](m:M) = {
        companions.find {
            case found:M => true
            case _ => false
        }
    }
}

abstract class Model {
    def test = {
        Holder.companionFor(this)
    }
}

abstract trait CompanionTrait[M <: Model] { self:Model =>
    Holder.companions = Holder.companions ++ Set(self)
}

class T1 extends Model {

}

object T1 extends Model with CompanionTrait[T1] {

}

class T2 extends Model {

}

object T2 extends Model with CompanionTrait[T2] {

}

T1
T2

val x = new T1
val y = new T2

println("testing")
assert(x.test == Some(T1))
println("passed 1")
assert(y.test != Some(T1))
println("passed 2")
assert(y.test == Some(T2))
println("passed 3")