Json serialization and polymorphism

[Aquachain]

Hello,

can someone tell me how to use Json serialization while preserving types. I use 
polymorphism and want to save and load data from an application.

Here is a small example:
    
    
    import
      json,
      marshal,
      os
    
    type
      Node* = ref object of RootObj
      InputNode* = ref object of Node
      OutputNode* = ref object of Node
    
    method signal*(this: Node) {.base.} =
      echo "wrong signal!"
    
    method signal*(this: InputNode) =
      echo "input signal"
    
    method signal*(this: OutputNode) =
      echo "output signal"
    
    var nodes: seq[Node]
    nodes.add(new InputNode)
    nodes.add(new OutputNode)
    
    echo "before JSON:"
    nodes[0].signal()
    nodes[1].signal()
    
    # serialization and deserialization
    let data = $$nodes
    nodes = to[seq[Node]](data)
    
    echo "\nafter JSON:"
    nodes[0].signal()
    nodes[1].signal()
    
    
    Run

The output is:
    
    
    before JSON:
    input signal
    output signal
    
    after JSON:
    wrong signal!
    wrong signal!
    
    
    Run

Im also happy if you can give me an alternative solution. Thanks in advance.