How do I enforce field type when saving through POST in MongoDB and Node.js? Aka, how to create a model

I'm trying to produce a 'catch-all' update POST query for my application.

Basically I need to perform the POST request with an identifier ID and fields to update, and it would update those fields dynamically. Here's what I have so far (in CoffeeScript):

router.post '/update', (req, res) ->
  whitelist = ['name', 'price', 'quantity', 'checked']
  toUpdate = {}
  for attr, val of req.body
    if attr in whitelist
      toUpdate[attr] = val

  if toUpdate.length > 0
    (req.db.collection 'products').findAndModify {
      query: {_id: req.body.id}
      update: {$set: toUpdate}
    }

Problem is, I can only pass it strings.

For example, if I wanna update a boolean, I don't know how to enforce the correct type.

POST request:

{ checked: 'true' }

Which will save like so in the database, but what I WANT to have is:

{ checked: true }

So my question is, is there a way to automatically enforce the correct type, so that it will convert by a set of rules I define per field?

Or do I have to code this manually before saving?

I tried looking up creating models in MongoDB, but it's really a big mess, I can't understand anything from the documentation. Google isn't coming up with good results either because I can't really seem to phrase what I want correctly in a search query.