rails virtual attributes won't read from form_for submission

问题:

I am trying to implement a rails tagging model as outlined in Ryan Bate's railscast #167. http://railscasts.com/episodes/167-more-on-virtual-attributes

This is a great system to use. However, I cannot get the form to submit the tag_names to the controller. The definition for tag_names is :

 def tag_names    @tag_names || tags.map(&:name).join(' ')  end

Unfortunately, @tag_names never gets assigned on form submission in my case. I cannot figure out why. SO it always defaults to tags.map(&:name).join(' '). This means that I can't create Articles because their tag_names are not there, and I also can't edit these tags on existing ones. Anyone can help?

回答1:

In short, your class is missing a setter (or in Ruby lingo, an attribute writer). There are two ways in which you can define a setter and handle converting the string of space-separated tag names into Tag objects and persist them in the database.

Solution 1 (Ryan's solution)

In your class, define your setter using Ruby's attr_writer method and convert the string of tag names (e.g. "tag1 tag2 tag3") to Tag objects and save them in the database in an after save callback. You will also need a getter that converts the array of Tag object for the article into a string representation in which tags are separated by spaces:

class Article << ActiveRecord::Base   # here we are delcaring the setter   attr_writer :tag_names    # here we are asking rails to run the assign_tags method after   # we save the Article   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

Solution 2: Converting the string of tag names to Tag objects in the setter

class Article << ActiveRecord::Base   # notice that we are no longer using the after save callback   # instead, using :autosave => true, we are asking Rails to save   # the tags for this article when we save the article   has_many :tags, :through => :taggings, :autosave => true    # notice that we are no longer using attr_writer   # and instead we are providing our own setter   def tag_names=(names)      self.tags.clear      names.split(/\s+/).each do |name|        self.tags.build(:name => name)      end   end    def tag_names     tags.map(&:name).join(' ')   end end

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