mgo を使用して MongoDB モデルに Go インターフェイスを埋め込むにはどうすればよいですか?

<code class="go">type Workflow struct {
   CreatedAt time.Time
   StartedAt time.Time
   CreatedBy string
   Nodes []Node
}

type Node interface {
  Exec() (int, error)
}</code>

<code class="go">w = &Workflow{}
collection.FindID(bson.ObjectIdHex(id)).One(w)</code>

<code class="go">type NodeWithType struct {
   Node Node `bson:"-"`
   Type string
}

type Workflow struct {
   CreatedAt time.Time
   StartedAt time.Time
   CreatedBy string
   Nodes []NodeWithType
}</code>

<code class="go">func (nt *NodeWithType) SetBSON(r bson.Raw) error {
    // Decode the node type from the "Type" field
    // Create a new instance of the concrete node type based on the decoded type
    // Unmarshal the remaining document into the concrete node type
    //...
}</code>