Active Record Relationship Design Patterns

In the world of web applications, eventually you’ve seen it all, and you start to see the same relational patterns occur over and over. To help out the newbies, here’s a list of various relationships (in Ruby on Rails syntax) that we see all the time.

Can you think of any more common relationship patterns?

Customer Relationship Management

class Company < ActiveRecord::Base
  has_many :contacts
end

class Contact < ActiveRecord::Base
  belongs_to :company
end

E-Commerce

class Category < ActiveRecord::Base
  has_many :products
end

class Product < ActiveRecord::Base
  belongs_to :category
end

class Order < ActiveRecord::Base
  has_many :line_items
end

class LineItem < ActiveRecord::Base
  belongs_to :product
  belongs_to :order
end

Roles Based Access Control

class User < ActiveRecord::Base
  has_and_belongs_to_many :roles
end

class Roles < ActiveRecord::Base
  has_and_belongs_to_many :users
  has_and_belongs_to_many :permissions
end

class Permission < ActiveRecord::Base
  has_and_belongs_to_many :roles
end

Mailing Lists:

class MailingList < ActiveRecord::Base
  has_many :subscribers
end

class Subscriber < ActiveRecord::Base
  belongs_to :mailing_list
end

Surveys / Questionaires / Quizzes:

class Survey < ActiveRecord::Base
  has_many :questions
  has_many :responses
end

class Question < ActiveRecord::Base
  belongs_to :survey
  has_many :choices
end

class Choice < ActiveRecord::Base
  belongs_to :question
end

class Response < ActiveRecord::Base
  belongs_to :survey
  has_many :response_choices
end

class ResponseChoice < ActiveRecord::Base
  belongs_to :response
  belongs_to :question
  belongs_to :choice
end

Hierarchical Content Management:

class Page < ActiveRecord::Base
  acts_as_tree
  acts_as_list :scope => :parent
end

Blogs:

class BlogEntry < ActiveRecord::Base
  has_many :comments, :dependent => true, :order => "created_at ASC"
end

class Comment < ActiveRecord::Base
  belongs_to :blog_entry
end

Social Networking:

(from: wiki.rubyonrails.com)

class User < ActiveRecord::Base
  has_and_belongs_to_many  :users,
                           :join_table => 'users_known_users',
                           :foreign_key => 'known_user_id',
                           :association_foreign_key => 'user_id',
                           :after_add => :create_reverse_association,
                           :after_remove => :remove_reverse_association

  def known_users
    self.users
  end

  private
    def create_reverse_association(associated_user)
      associated_user.known_users << self unless associated_user.known_users.include?(self)
    end

    def remove_reverse_association(associated_user)
      associated_user.known_users.delete(self) if associated_user.known_users.include?(self)
    end
end

Any major ones I’m missing here?