What's New in Edge Rails: Touching

Posted by ryan
at 7:51 AM on Monday, April 20, 2009
This feature was released in Rails v2.3.3

There are often times when you want an update made to one object to be reflected up the object graph as an update of an associated parent object. For instance, if a new comment is created on an article, you may very well want to mark the article as being updated. With the new touch feature of ActiveRecord, this is a whole lot easier. Using our previous example, here’s is how it works:

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class Article < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base

  # Make create/update/deletes of a comment mark its
  # parent article as updated
  belongs_to :article, :touch => true
end

# Adding a new comment marks the article as being updated
article.updated_at #=> "Mon Apr 20 07:42:53 -0400 2009"
article.comments.create(:body => "New comment")
article.updated_at #=> "Mon Apr 20 07:43:27 -0400 2009"

# Same for updates/deletes
article.comments.first.destroy
article.updated_at #=> "Mon Apr 20 07:45:23 -0400 2009"

This is a great way to keep tightly coupled domain models in-sync without resorting to a potential maze of callback logic.

Also, if you have a timestamp field named something other than the standard updated_at or updated_on you can explicitly specify that field as the value to the :touch option and it will get marked instead:

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class Article < ActiveRecord::Base
  has_many :comments
  validates_presence_of :last_updated_at  # non-standard
end

class Comment < ActiveRecord::Base
  belongs_to :article, :touch => :last_updated_at
end

# Adding a new comment marks the article as being updated
article.last_updated_at #=> "Mon Apr 20 07:42:53 -0400 2009"
article.comments.create(:body => "New comment")
article.last_updated_at #=> "Mon Apr 20 07:43:27 -0400 2009"

Also, somewhat conveniently, you can invoke touch directly on a model to update its timestamp outside any association callbacks:

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article.updated_at #=> "Mon Apr 20 07:42:53 -0400 2009"
article.touch
article.updated_at #=> "Mon Apr 20 07:43:27 -0400 2009"

So, touch away (in a non-creepy kind of way)!

tags: ruby, rubyonrails