A quick hint, on how to have an ‘id’ attribute in Mongoid document JSON:

When you serialize Mongoid objects to JSON, you won’t get ID attribute, similar to ActiveRecord. Instead you’ll have “_id” attribute containing “$oid” value:

{
  "_id": {"$oid":"52d3fdfd53656e5180020000"},
  "changed_at":"2014-02-15",
  "created_at":"2014-01-13T15:53:49.444+01:00",
  "updated_at":"2014-01-13T15:53:49.444+01:00"
}

If you would prefer something like this:

{
  "id": "52d3fdfd53656e5180020000",
  "_id": {"$oid":"52d3fdfd53656e5180020000"},
  "changed_at":"2014-02-15",
  "created_at":"2014-01-13T15:53:49.444+01:00",
  "updated_at":"2014-01-13T15:53:49.444+01:00"
}

Just put this into your initializers (for example in config/initializers/mongoid.rb):

module Mongoid
  module Document
    def as_json(options={})
      attrs = super(options)
      attrs["id"] = attrs["_id"].to_s
      attrs
    end
  end
end