Скачать или смотреть How to Use the after_update Callback to Track Changes in Ruby on Rails

Discover how to effectively track previous values of attributes in Ruby on Rails using the `after_update` callback. Learn best practices for sending notifications based on status changes.
---
This video is based on the question https://stackoverflow.com/q/76931052/ asked by the user 'Ben' ( https://stackoverflow.com/u/9434505/ ) and on the answer https://stackoverflow.com/a/76931919/ provided by the user 'Alex' ( https://stackoverflow.com/u/207090/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Which callback must I use to get previous value?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding Callbacks in Ruby on Rails

When working with Ruby on Rails, especially when dealing with model attributes, you may encounter scenarios where you need to track changes between previous and new values. This often comes into play when you want to execute certain actions based on these changes, like sending notifications when a record is published.

The Problem

Suppose you're developing a feature for a shop where you need to send an email notification whenever an advertisement's status changes from anything other than "published" to "published". You may find yourself confused about the correct callback to use for retrieving the previous state of an attribute during an update. For example:

[[See Video to Reveal this Text or Code Snippet]]

Here, you want to check if the previous status was not "published" and then trigger a notification.

What Went Wrong?

In a Rails controller, if you use a before_action callback, the object's attributes have not been updated yet, which can lead to inconsistencies when checking the values:

[[See Video to Reveal this Text or Code Snippet]]

You would expect @ ad.status to be "published" after you assign new parameters, but this change only reflects after the update action is executed.

The Solution: Using the Proper Callbacks

To achieve the desired functionality, you have several strategies available, depending on where you want to handle this logic - in the controller or the model.

1. Using after_update Callback

If you prefer to handle changes at the model level, the after_update callback in the model is the most appropriate choice. This ensures that you can reliably use status_was and status_previously_was to check for previous values only after the changes have been saved.

Example:

[[See Video to Reveal this Text or Code Snippet]]

2. Handling in Controller's Update Action

If you opt to manage this in the controller, using the after_action is more suitable. This ensures you have confirmed that the ad has been updated successfully before checking the status:

Example:

[[See Video to Reveal this Text or Code Snippet]]

Summary of Key Points

Use after_update in models to check status_was or status_previously_was for reliable state tracking.

Utilize after_action in controllers to ensure modifications are confirmed before checking for changes.

Remember that status_was reflects the last saved value before the most recent change, whereas status_previously_was reflects the value before the last update was made.

By following these guidelines, you can effectively manage attribute changes in Ruby on Rails and send notifications based on specific criteria.