RealmSwift: issue whilst saving relationship objects with primary key

I'm trying out Realm and ObjectMapper.

I have these two models:

class User : Object, Mappable {

    override class func primaryKey() -> String {
        retu "id"
    }

    dynamic var id : String

    [...]

    func mapping(_ map: Map) {
        id <- map["id"]
    }

}

class Media, Object,  Mappable {

    override class func primaryKey() -> String {
        retu "id"
    }

    dynamic var id : String
    dynamic var owner : VPUser?

    [...]

    func mapping(_ map: Map) {
        id <- map["id"]
        owner <- map["owner"]
    }

}

Login is fine: the JSON gets mapped correctly and the realm insertion goes fine as well.

The issue arises when I fetch the media remotely. Each media object contains an user object (owner).

This is how the media JSON looks like:

{
      "id": "574cd4e6e4b0b7e43219664a",
      "owner": {
          "id": "574cd462e4b0b7e432196644",
       }
}

Now, the User object with that id may exists already, therefore I get this exception:

Can't set primary key property 'id' to existing value...'

Note: I'm using Realm.create(_, update: true) to save the media.

Any clue on how to solve it? Cheers.