users.rb

class User
  # Return a string suitable for uniquely identifying this user in URLs
  # @return [String]
  def to_param
    begin
      n = shadow_username
      n = _id.to_s if n.blank? or (username_changed? and !valid?)
      n
    end
  end

  # Find a single record by ID or slug
  # @param [username, document ID] Username or document ID to find
  # @raise MongoMapper::DocumentNotFound
  # @return [Integer]
  def get!(id)
    id = id.to_s.downcase
    if id.match(/[a-f0-9]{24,}/i)
      find(id)
    else
      where(:username => id).find
    end
  end
end

# Usage:

user_path(@user) # => /users/508bec6597b2f809b6000001
user_path(@user) # => /users/cheald

User.get!("508bec6597b2f809b6000001")
User.get!("cheald")