mongoose casting object ids of type string to ObjectId()

gistfile1.js

var mongoose = require('mongoose')
  , db = mongoose.createConnection('localhost', 'test')

var CommentSchema = mongoose.Schema({
  title : 'string'
})

var PostSchema = mongoose.Schema({ 
  name: 'string'
  , comments : [CommentSchema]
})
var Post = db.model('Post',PostSchema)

Post.collection.drop()

var post = new Post({ 
  name : 'some post'
  , comments : [
    {
      title : 'Comment 1'
    }
    , {
      title : 'Comment 2'
    }
  ]
}).save(function(err, post){
  if(err) throw err
  var post_id = post._id
    , comment_id = post.comments[0]._id.toString()
  console.log('post')
  console.log(post)
  Post.update( 
    // id of the post to update
    { _id : post_id }
    // the `update` query to run
    , {
      $pull : {
        comments : { _id : comment_id }
      }
    }
    // the callback
    , function(err, numEffected){
      if(err) throw err
      console.log('numEffected')
      console.log(numEffected)
      Post.findById(post_id, function(err, post){
        console.log(post)
        process.exit()
      })
  })
})