In my solution I have to do the following:
users = CloudUser.all
present :data, users, :with => CloudUser::Entity
class CloudUser < ActiveRecord::Base
self.primary_key = :username
has_many :configurations, :class_name => 'Configuration'
has_many :favorites, :class_name => 'Favorite', :foreign_key => :username
class Entity < Grape::Entity
expose :username, :email, :first_name, :last_name
So it wasn't as easy as just defining a class Entity for me.