# File lib/mongo/db.rb, line 178
    def add_user(username, password, read_only = false)
      users = self[SYSTEM_USER_COLLECTION]
      user  = users.find_one({:user => username}) || {:user => username}
      user['pwd'] = Mongo::Support.hash_password(username, password)
      user['readOnly'] = true if read_only;
      users.save(user)
      return user
    end