Call populate(path, fields, conditions, options) with options.suffix
defined, and the populated object will go into a new field, key+suffix,
instead of overwriting the id field.

For example, for compatibility with ExtJS4, we might want to send down
an object in this manner to be friendly to ExtJS' association support:

User{
   comment: "4f5cd5f6e9786ec113000006"
   comment_obj: {text: "foo"}
}

Calling User.findOne().populate('comment', null, null, {suffix:
'_obj'}) will add this suffix.