Question

I’m trying to write a library that will programmatically add around_update/around_destroy callbacks to a ActiveRecord model.

So, a regular model would look like this and it works as expected:

class User < ActiveRecord::Base
  around_update :test_update

  def test_update
    Rails.logger.debug "test_update"
    yield
    Rails.logger.debug "Finished test_update"
  end
end

u=User.last
u.name = 'something'
u.save

######### output (as expected):
# test_update
# Finished test_update

My little library (obviously just the skeleton) looks like this:

# A module for creating around callbacks in a model
module Piddle
  module TimelineFor
    def self.included(klass)
      klass.send(:extend, ClassMethods)
    end

    module ClassMethods
      def timeline_for(event, opts={})
        method_name = :"timeline_for_#{event.to_s}"
        define_method(method_name) do |&block|
          Rails.logger.debug method_name.to_s
          yield block
          Rails.logger.debug "After yield in #{method_name.to_s}"
        end

        send(:around_update, method_name)
      end
    end
  end
end

It defined a timeline_for method which should add the timeline_for_update method and make that a callback for the around_update event. And the User model I want to use is this:

# second version of the User model using Piddle to create the callback
require 'piddle/piddle'

class User < ActiveRecord::Base
  include Piddle::TimelineFor

  timeline_for :update
end

u=User.last
u.name = 'gfgfhfhfgh'
u.save

In the output I see

timeline_for_update
LocalJumpError: no block given (yield)
from /vagrant/lib/piddle/piddle.rb:13:in `block in timeline_for'

The first output line indicates that the method is being called but the block isn’t being passed in.

Any ideas or alternative implementations?

Answer 1

The problem is that if you call yield from your define_method, ruby interprets that as trying to yield to the (nonexistant) block that was passed to timeline_for, not the block
that rails passed to timeline_for_foo

You’ve got block being passed to you so you can just call it:

def timeline_for event
  method_name = "timeline_for_#{event}"
  define_method method_name do |&block|
    ActiveRecord::Base.logger.debug "before #{method_name} yield" 
    block.call
    ActiveRecord::Base.logger.debug "after #{method_name} yield" 
  end
  send :around_update, method_name.to_sym #must use a symbol here
end