Carnegie Mellon Example Rails Model

wtf.rb

class Assignment < ActiveRecord::Base
  # Relationships
  belongs_to :user
  belongs_to :project
  
  # Scopes
  scope :active, where('assignments.active = ?', true)
  scope :inactive, where('assignments.active = ?', false)
  scope :for_project, lambda { |project_id| where('project_id = ?', project_id) }
  scope :for_user,    lambda { |user_id| where('user_id = ?', user_id) }
  scope :by_user,     joins(:user).order('users.last_name, users.first_name')
  scope :by_project,  joins(:project).order('projects.name')
  
  # Validations
  validate :user_is_active_in_system
  validate :project_is_current_in_system, :on => :create
  
  private
  def user_is_active_in_system
    all_active_users = User.active.all.map{|u| u.id}
    unless all_active_users.include?(self.user_id)
      errors.add(:user_id, "is not an active user in the system")
    end
  end
  
  def project_is_current_in_system
    all_current_projects = Project.current.all.map{|p| p.id}
    unless all_current_projects.include?(self.project_id)
      errors.add(:project_id, "is not a current project in the system")
    end
  end  
end