Bogdan Gusiev's blog

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New Rails Release with a few features from me
23 Jan 2012

Upcoming Rails 3.2 release has many useful features.

Some of them are coming from me:

  • ActiveRecord #pluck method
  • Strict validation concept
  • Customizable mass assignment sanitizer behavior
  • Gotcha for select[multiple] tag

Here is a more detailed description than in the Changelog.

Pluck method

People around made a lot of PR on it. So, You probably already heard about it.

pluck can be used to query a single column from the underlying table of a model. It accepts a column name as argument and returns an array of values of the specified column with the corresponding data type.

Client.where(:active => true).pluck(:id)
# SELECT id FROM clients WHERE active = 1


Pluck in Rails guides

Strict validation concept

Some validation are not connected with end user and breaking it is more likely a bug than problem with user input. A few constraints can be embed to DB(like foreign keys), but most of them can not. And it’s a good idea to use AM validators to do such checks as well.

Use case:

class Article < AR::Base
  validates :author, :presence => true

u = # => false

#author is usually set in controller from current_user. If somebody forgets to do it - user see error: Author cann't be blank. And it is not right to show error message to user that he is not able to fix. More over DEV team is not notified about the problem.

In order to fix that You can use validate! method that generates validator that always raises exception when fails:

class Article < AR::Base
  validates! :author, :presence => true

u = # raises ActiveModel::StrictValidationFailed

Customizing Mass Assignment Sanitizer

Added an ability to specify your own behavior on mass assignment protection, controlled by option:


The main idea behind this change is to make Mass Assignment Sanitizer to raise exceptions in development and test environments while leaving logging behavior in production. This will be the default Rails application template in version 3.2. It will allow to detect mass assignment protection issues caused by bug in code more effectively before they will be deployed to production.

Power hacking select[multiple]

Quote from Rails documentation that comes along with the patch:

The HTML specification says:

When multiple parameter passed to select and all options got deselected web browsers do not send any value to server. Unfortunately this introduces a gotcha: if an User model has many roles and have role_ids accessor, and in the form that edits roles of the user the user deselects all roles from role_ids multiple select box, no role_ids parameter is sent. So, any mass-assignment idiom like


wouldn’t update roles.

To prevent this the helper generates an auxiliary hidden field before every multiple select. The hidden field has the same name as multiple select and blank value.

<input type="hidden" name="user[role_ids]" value=""/>
<select id="user_role_ids" name="user[role_ids]" multiple="multiple">

This way, the client either sends only the hidden field (representing the deselected multiple select box), or both fields. Since the HTML specification says key/value pairs have to be sent in the same order they appear in the form, and parameters extraction gets the last occurrence of any repeated key in the query string, that works for ordinary forms.

Unfortunately this trick can introduce a side effects described well in this stackoverflow question. If you know an effective way to fix that - let me know.

Have fun with upgrading

rails changelog contribution