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Classmeta for Rails 3.2+

Creates new classes on the fly to allow you to refactor common changes.

Lets say you wanted whitelisted attributes to be different depending which class was including it as an association.

In car, you only want the names of passengers:

has_many :passengers, class_name: Person.meta(:std, attrs: [:name]).name

In train, you want their seat preference:

has_many :riders, class_name: Person.meta(:std, attrs: [:name, :seat_preference]).name

So, include the gem in Gemfile and bundle install:

gem 'classmeta'

Add a transformer somewhere in the load path like app/models/:

class StandardTransformer
  def transform(klazz, options)
    klazz.class_eval "self._accessible_attributes[:default] = #{options[:attrs].inspect}" if options[:attrs]

And add this to environment.rb:

  :transformers => {
    :std => StandardTransformer

Now magically car passengers have their accessible attributes set to :name and train riders to [:name, :seat_preference]!


Hooking into Rails' load_missing_constant, duping the original class, and doing stuff to it in the transform.

Works with class reloading or caching.


It's useful if you have a number of classes, like models, that just differ by a little bit and module includes just aren't solving the problem of doubling, tripling, etc. the number of files you are having to create just to represent new classes.

Just Rails?

Feel free to add off-Rails support and do a pull request.

Quick Test!

Add to your Gemfile:

gem 'classmeta'

Install it:

bundle install

Open console:

rails c

Try it:

YourModel.meta(:echo, {:say => 'Hello World!'})
YourModel.named_meta('Fabular', :echo, {:say => 'Hello World!'})

Note: if you define Classmeta::Options.configure({...}), it automatically gets rid of the echo transformer which is just for demonstration purposes. If you want to use it at runtime, you can use:

  :transformers => {
    :echo => Classmeta::Echo


Since Classmeta delegates to Rails's dependency loading, any errors you get like this:

.../classmeta/lib/classmeta/dependencies.rb:7:in `load_missing_constant': uninitialized constant (something) (NameError)

Just means Rails couldn't find your class.

For other errors, too, for the most part, pretend you are getting that from .../activesupport/lib/activesupport/dependencies.rb, then Google it and scratch your head until you figure out you mistyped something.


Copyright (c) 2012 Gary S. Weaver, released under the MIT license.

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