Rails - Jul 2006 - Using validators for appending to a HABTM association

class User < ActiveRecord::Base
  has_and_belongs_to_many :projects
end

class Project < ActiveRecord::Base
  has_and_belongs_to_many :users
end

class ProjectController < ApplicationController
  def add_user
    @project = Project.find(params[:project_id])
    @user = User.find(params[:id])
    @project.users << @user unless @project.users.include?(@user)
    # I''d honestly rather do the above with a validator instead of a
condition
    # This way I can show the error to the person using the error
validators I have now
    # What would be the best way of doing this?
    # Thanks!
  end
end