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Validates Constancy for Ruby on Rails (Active Record)

Compatible with Rails v1.2.2 through v2.0.4 (Active Record v1.15.2 through v2.0.4)


This 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 a Rails plugin. It is compatible with the latest released version of the Rails framework (and possibly also other versions – see the 'test' branch of this repository). You can install the plugin with the command:

ruby script/plugin install git://

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



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 => { |comment| comment.locked? }


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?'


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


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 “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.

git checkout test
git submodule update --init

Then read rails_*/doc/README_FOR_APP for instructions on how to run the tests.


Copyright © 2007-2009 Nils Jonsson (

Released under the MIT license.

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