does it possible contains multi format document in one lift-mongo-recorder model?

Question

I attempt build a model able to insert different field(only one field is different).
also, the field should be custom format and I will be a embed object. document form Assembla use case class define embed field, so I can't use base class(trait/abstract class) to define under my recorder model.

hope insert different format, such as

{
  _id: ...,
  same_field: "s1",

  specify_field: {
    format1: "..."
  }
}

{
  _id: ...,
  same_field: "s1",

  specify_field: [
    formatX: "..",
    formatY: ".."
  ]
}

How to construct my model?
thanks