#59 Optimistic Locking (revised)

Nice idea. It should be in rails like built-in validation. Thank's Ryan.

Sidequestion: Is it better to load modules into my models? I currently use abstract models for most things (which, however, loads modules itself). But I runned into problems as the self.abstract_class = true gotcha seems to be not widely known...

The video is not working :(

Thanks so much as always! There is an episode for nearly every thing I want to do! Amazing!

Using the second method, I had a behavior with a remote form that I wanted to pass by the group. I got the first validation error just fine. If I hit submit again, however, the old value would pass the validation the second time and overwrite the value that was protected the first time around.

In other words, the conflict was not detected because @original_updated_at was set to nil and the object reloaded the new timestamp and passed the update right through. If I comment out the @original_updated_at = nil line, I get the behavior I want -- a durable block on any updates to that record.

Does this make sense? What was the value of setting @original_updated_at to nil in the code? Thanks for any advice you might have! Charles

One more quick discovery/question. With the second method, I was getting spurious locking errors added to base on creation. It turned out acts_as_list and other such plugins intercept the create process and perform some quick saves to get things in the right place. Not sure I completely understand, but I needed to pass the validation if nothing was in the changes array:

# app/models/item.rb
def handle_conflict
    if @conflict || updated_at.to_f > original_updated_at.to_f
      
      # Commented out because of ability to submit again and overwrite previous value
      # @original_updated_at = nil    
      
      # acts_as_list and other items update the position and cause spurious conflicts during creation
      if changes.empty?
        @conflict = false
      else
        @conflict = true
        errors.add :base, "Snap! Someone signed up for this just a moment ago. Pick something else?"
        changes.each do |attribute, values|
          errors.add attribute, "has just been reserved. Cancel to reload sheet."
        end
      end
    end
  end

This got my simple creation action up and running again. Thanks for this great episode and any insights on whether this is a short sighted fix. Charles

One thing not mentioned in the episode is that HTTP reserves the 409 status code to indicate conflicts. render status: :conflict also works. Set it for your response, especially in APIs.

Hey Ryan, great screencast but I have one question. Say the user updates the value of the hidden field in the DOM for some weird reason (using inspect element). How can we account for this?

Great idea; works very well, is elegant and is set at the right places.

This was yet another awesome tutorial. I have chosen to go with your custom locking mechanism in my current project. Although I had to figure out my way through using an added time based field in the database as you mentioned at the end of the video, I did make one change to your handle_conflict method, changing the line that reads:

errors.add name, "was #{values.first}"

to:

errors.add attribute, "was " + (values.first.blank? ? "blank" : values.first.to_s)

To handle cases where something was added to a blank field during edit.

By means of the subsequent technique, I had a performance with a distant shape that I required to go by by the collection. I got the primary corroboration mistake just well.

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hey dear thanks for posting this good information yes it can happen that one can override other can you give us a brief demo of optimized unlocking

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