Overwriting a function prototype

protype.js

var util = require("util");

//// Model ////

var Model = function(data) {
	this._data = data
}

Model.prototype.show = function (data, cb) {
	// act on data and then return exicute the callback
  // like pulling something from the database
  cb(this._data[data.user])
}

/// Collections Model

var collectionModel = function (data) {
  Model.call(this, data)
}

util.inherits(collectionModel, Model)

/// Extending the model

collectionModel.prototype.show = function (data, cb) {
  var _show = this.show

  _show(data, function (data) { // TypeError: undefined is not a function
    data.name = data.name.toUpperCase();
    cb(data);
  })

}

var data = [
    {
      name: 'Travis'
    }
  , {
      name: 'John'
    }
  , {
      name: 'Sarah'
    }
  ]

//
// General model
//

model = new Model(data)

model.show({
  user: 1
}, function (data) {
  console.dir(data)
})

//
// Collection model, returns name in uppercase
//

collection = new collectionModel(data)

collection.show({
  user: 1
}, function (data) {
  console.dir(data)
})