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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.