#189 Embedded Association

Nov 23, 2009 | 14 minutes | Active Record, Views

Learn how to set up a one-to-many or many-to-many association which is entirely embedded into a single column through a string or bitmask.

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One to Many

          bash
        
script/generate migration add_role_to_users role:string
rake db:migrate

          models/user.rb
        
class User < ActiveRecord::Base
  acts_as_authentic
  has_many :articles
  has_many :comments
  
  ROLES = %w[admin moderator author]
  
  def role_symbols
    [role.to_sym]
  end
end

          users/new.html.erb
        
<p>
  <%= f.label :role %><br />
  <%= f.collection_select :role, User::ROLES, :to_s, :titleize %>
</p>

Many to Many

          bash
        
script/generate migration add_roles_mask_to_users roles_mask:integer
rake db:migrate

          models/user.rb
        
class User < ActiveRecord::Base
  acts_as_authentic
  has_many :articles
  has_many :comments
  
  named_scope :with_role, lambda { |role| {:conditions => "roles_mask & #{2**ROLES.index(role.to_s)} > 0"} }
  
  ROLES = %w[admin moderator author]
  
  def roles=(roles)
    self.roles_mask = (roles & ROLES).map { |r| 2**ROLES.index(r) }.sum
  end
  
  def roles
    ROLES.reject { |r| ((roles_mask || 0) & 2**ROLES.index(r)).zero? }
  end
  
  def role_symbols
    roles.map(&:to_sym)
  end
end

          users/new.html.erb
        
<p>
  <%= f.label :roles %><br />
  <% for role in User::ROLES %>
    <%= check_box_tag "user[roles][]", role, @user.roles.include?(role) %>
    <%=h role.humanize %><br />
  <% end %>
  <%= hidden_field_tag "user[roles][]", "" %>
</p>