SingletonValidations
===================

David A. Black
Ruby Power and Light, LLC

Version 1.0.0
July 4, 2008

This plugin makes it possible to use the ActiveRecord class-level validation
methods at the instance level. You can call methods like validates_size_of and
validates_format_of directly on your Thing objects. These "singleton"
validations will override the class ones, if those are present. 

The idea for this plugin came from an IRC question by 'mickey', about how to get
class-style validations on a singleton basis. I haven't found mickey again. But
I appreciate his or her getting me thinking about this. 


Example
=======

Here's a simple "thing" model, followed by some examples of how to use the
validates_* class methods on individual objects of the Thing class. 

things table:
  name        string
  priority    integer
  description string

class Thing < ActiveRecord::Base
  validates_size_of :name, :minimum => 3
  validates_numericality_of :priority
end

t = Thing.new(:name => 'abc', :priority => 10)
p t.save                  # true

t.validates_size_of :name, :minimum => 5
p t.save                  # false
p t.errors.entries        # name is too short, minimum 5

t.priority = 10.1
t.validates_numericality_of :priority, :only_integer => true
p t.valid?                # false
p t.errors.entries        # name too short; priority is not a number

t.priority = 10
t.name = "abcde"
p t.valid?                     # true

t.validates_presence_of :description
p t.valid?                     # false; no description

t.description = "abc"
p t.valid?                     # true

# Just to make sure that the singleton validations didn't
# propagate to the class: try a new Thing object. 

t = Thing.new
p t.valid?                     # false
p t.errors.entries


How it works
============

The plugin works by overriding the run_callbacks method in the
ActiveSupport::Callbacks module, so that it checks not only the object's class
but also its singleton class for callback chains. The callback chains get
inserted into the singleton class of an object courtesy of
ActiveRecord#method_missing, which has been hooked with alias_method_chain so
that when you call a method that starts with "validates_", it checks to see
whether that method is a class method and if it is, it runs it in the singleton
class context. 


License and (lack of) warranty
==============================

MIT License (see file).