Helper with dynamic dependent key... possible?

1

I’m writing a toSentence helper that will take an array of items (say, people) and an attribute name (say firstName), and build a “sentence” out of it. For example:

With

people = [Em.Object.create(firstName: 'Pedro'), Em.Object.create(firstName: 'Pablo'), Em.Object.create(firstName: 'Diego')]

then

{{toSentence people attr=firstName}}

outputs

Pedro, Pablo and Diego

This works until one of the object’s firstName (or whatever attribute) changes, since the helper has no dependent keys. To fix this, when defining the helper I would have to add the dependent keys, like so:

Ember.Handlebars.helper 'toSentence', (thefunction), '@each.firstName'

But this ONLY works for the firstName attribute… How can I make that key “dynamic”? Or am I approaching this the wrong way?

For completion’s sake, here’s the entire toSentence helper:

Ember.Handlebars.helper 'toSentence', (items, options) ->
  array = items.toArray().map (item) ->
    if options.hash.attr?
      Handlebars.Utils.escapeExpression(item.get(options.hash.attr))
    else
      Handlebars.Utils.escapeExpression(item.toString())

  sentence = if array.length == 0
    ""
  else if array.length == 1
    "#{array[0]}"
  else if array.length == 2
    "#{array[0]}, #{array[1]}"
  else
    "#{array[0..-2].join(', ')} and #{array[array.length-1]}"

  new Handlebars.SafeString(sentence)