Set reference id fields early

[erikmuttersbach]

We should solve the following problem

let m = new Model()
m.referencedModel = new ReferencedModel()
assert(m.referencedModel)       // true 
assert(m.referencedModel_id)  // false -> should be true
await m.referencedModel.save()
assert(m.referencedModel)       // true 
assert(m.referencedModel_id)  // true

let m = Model.get<Model>(...)
assert(m.referencedModel)       // true 
assert(m.referencedModel_id)  // true
await m.referencedModel.save()
assert(m.referencedModel)       // true 
assert(m.referencedModel_id)  // true