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Observers: observing when changes have been made...

Posted by Annalisa Afeltra in Ruby on Rails - 3 comments

I wanted to find a solution that only sends an email to notify when a change has been made to the page. I came across the Observer. It works great when you need to compare if the value have been changed from before the save button to after.

This is how it works:

In the controller, add the following at the top of the page:

observer :mail_observer

and in the model, add the following to get the changed attributes:

attr_accessor :changed_attributes

Create a new model:

mail_observer.rb

Your mail_observer should look similar to this, I call two callback methods before_save and after_save:

class MailObserver < ActiveRecord::Observer
  #observe and the name of your model
  observe Task 

  def before_save(model)

   #Place the old values into a hash table only if it is not a new record being inserted
    old_attr = Hash.new
    if !model.new_record?
      old_model = model.class.find(model.id)
      old_attr = old_model.attributes
    end

    tmp_diff = Hash.new

    #delete the values in the hash table where the values have not changed, leaving you with the changed values
    model.attributes.delete_if do |key,value|
      old_attr[key] == value    
    end.keys.each do |k|
      tmp_diff[k] = old_attr[k]
    end

    model.changed_attributes = tmp_diff   
  end

In the after_save function I check which values have changed by !model.changed_attributes[‘resource_name’].nil?, for example if a task that has been assigned to a person, is changed to another person, an email is sent to alert the new person that a task has been assigned to them.

  #the after save function you can check which values have been changed and do something for it, depending on what has changed

  def after_save(model)      
      #task resource_name
      if !model.changed_attributes['resource_name'].nil?
         #send an email to the person that has been assigned this new task
         puts 'the owner of this task is has been changed.' 
      end
 end

This could be a good example to check when changes have been made to a page and to alert the changes that have been made.

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