How to create field using variable name?

How can I create a field in mongoose using a variable name? I've seen some ways to do it using .update(), but I was wondering if there was a way to do it when creating a new document

I have my schema like:

var summariesSchema = mongoose.Schema({
  type: String,
  name: String,
  date: String
})

and my object:

var date = '2015-02-01'
var obj = {
  ios: 100,
  android: 500
}
var doc = {}
doc[date] = obj
var mongoDoc = new Summaries({
  name: 'John',
  type: 'person',
  date: date,
  $doc: doc
})
mongoDoc.save(function(err){
  if(!err) log('done')
  else log(err.toString())
})

But it only saves the fields 'name', 'type' and 'date'.

Can anyone tell me if its possible to do something like that and if so, what am I missing?