#167 More on Virtual Attributes

Jun 22, 2009 | 7 minutes | Models, Forms

Use a virtual attribute to implement a simple tagging feature. In this episode I show you how to assign virtual attributes through a callback instead of a setter method.

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          bash
                
        script/generate model tag name:string
script/generate model tagging article_id:integer tag_id:integer
rake db:migrate
        
script/generate model tag name:string
script/generate model tagging article_id:integer tag_id:integer
rake db:migrate

          models/tagging.rb
                
        class Tagging < ActiveRecord::Base
  belongs_to :article
  belongs_to :tag
end
        
class Tagging < ActiveRecord::Base
  belongs_to :article
  belongs_to :tag
end

          models/tag.rb
                
        class Tag < ActiveRecord::Base
  has_many :taggings, :dependent => :destroy
  has_many :articles, :through => :taggings
end
        
class Tag < ActiveRecord::Base
  has_many :taggings, :dependent => :destroy
  has_many :articles, :through => :taggings
end

          models/article.rb
                
        class Article < ActiveRecord::Base
  has_many :comments, :dependent => :destroy
  has_many :taggings, :dependent => :destroy
  has_many :tags, :through => :taggings
  validates_presence_of :name, :content
  attr_writer :tag_names
  after_save :assign_tags
  
  def tag_names
    @tag_names || tags.map(&:name).join(' ')
  end
  
  private
  
  def assign_tags
    if @tag_names
      self.tags = @tag_names.split(/\s+/).map do |name|
        Tag.find_or_create_by_name(name)
      end
    end
  end
end
        
        
        
        
        
        
        
        
        
      
class Article < ActiveRecord::Base
  has_many :comments, :dependent => :destroy
  has_many :taggings, :dependent => :destroy
  has_many :tags, :through => :taggings
  validates_presence_of :name, :content
  attr_writer :tag_names
  after_save :assign_tags
  
  def tag_names
    @tag_names || tags.map(&:name).join(' ')
  end
  
  private
  
  def assign_tags
    if @tag_names
      self.tags = @tag_names.split(/\s+/).map do |name|
        Tag.find_or_create_by_name(name)
      end
    end
  end
end

          articles/_form.html.erb
                
        <p>
  <%= f.label :tag_names %><br />
  <%= f.text_field :tag_names %>
</p>
        
<p>
  <%= f.label :tag_names %><br />
  <%= f.text_field :tag_names %>
</p>