Can't Delete Behaviour

I am having trouble with something that should be patently obvious. I cant seem to be able to delete properties of my object in node. This is in coffeescript, but I don't think that is relevant.

console.log doc
delete doc.password
console.log doc

Returns:

{ firstname: 'Andrew',
  lastname: 'Plummer',
  email: 'andrew@stackoverflow...',
  password: 'blahblah',
  _id: 5014c0a6af36bdaf03000001,
  comments: [],
  posts: [] }

{ firstname: 'Andrew',
  lastname: 'Plummer',
  email: 'andrew@stackoverflow...',
  password: 'blahblah',
  _id: 5014c0a6af36bdaf03000001,
  comments: [],
  posts: [] }

For something so seemingly obvious I have actually searched around stackoverflow and googled for about half an hour. Sorry if still i have missed a duplicate.