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Validates Constancy

Filed under ActiveRecord
Developed by Nils Jonsson (njonsson)

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Introduction

This RubyGem and Rails plugin adds a validates_constancy_of validation to Active Record. It allows you to prevent particular database fields from being changed after a record is created. A validation error occurs on updates if an attribute of a model object is different from its value in the database.

Installing Validates Constancy

The code is packaged as both a RubyGem and a Rails plugin. You can use either one, depending on what your needs are.

The Validates Constancy gem is compatible with various versions of Rails (Active Record)—see the test subdirectories of constancy.rubyforge.org/svn/gem/branches. You can install the gem with the command:

gem install validates_constancy

The Validates Constancy plugin is compatible with the latest released version of the Rails framework (and possibly also other versions—see constancy.rubyforge.org/svn/plugin/test). You can install the plugin with the command:

ruby script/plugin install http://constancy.rubyforge.org/svn/plugin/validates_constancy

Use the gem (constancy.rubyforge.org/svn/gem) if you’re using Active Record apart from Rails, or for compatibility with a version of Rails (Active Record) that is not supported by the plugin. Use the plugin if your Rails version is up to date and if you like the convenience of a Rails plugin.

Using constancy validation

Here’s how to use this validation in your code.

class Person < ActiveRecord::Base

  # Prevent changes to Person#social_security_number.
  validates_constancy_of :social_security_number

end

Options

The validation takes two options, :if and :message. These may be familiar because several of Active Record’s validations also use them. The :if option takes a Proc, or a symbol, or string with a model object argument and a return value of true or false.

class Comment < ActiveRecord::Base

  # Prevent changes to Comment#text if it is "locked." 
  validates_constancy_of :text, :if => Proc.new { |comment| comment.locked? }

end

The default error message is “can’t be changed”. Use your own error message by specifying the :message option.

class LicensePlate < ActiveRecord::Base

  # Prevent changes to LicensePlate#number.
  validates_constancy_of :number,
                         :message => 'is off-limits! What are you thinking?'

end

More than one model attribute can be specified. Any specified options will be applied to all the specified attributes.

Warning

With associations, validate the constancy of a foreign key, not the instance variable itself: validates_constancy_of :invoice_id instead of validates_constancy_of :invoice.

Also note the warning under Inheritable callback queues in api.rubyonrails.org/classes/ActiveRecord/Callbacks.html. “In order for inheritance to work for the callback queues, you must specify the callbacks before specifying the associations. Otherwise, you might trigger the loading of a child before the parent has registered the callbacks and they won’t be inherited.” Validates Constancy uses these callback queues, so you’ll want to specify associations after validates_constancy_of statements in your model classes.

Running automated tests for Validates Constancy

There’s a suite of tests that exercises all the functionality of Validates Constancy. You can check out a version of the test suite from the repository according to the version of Rails (Active Record) it works with.