ActsAsEavModel
==============

ActsAsEavModel allow for the Entity-attribute-value model (EAV), also 
known as object-attribute-value model and open schema on any of your ActiveRecord
models. 

= What is Entity-attribute-value model?
Entity-attribute-value model (EAV) is a data model that is used in circumstances 
where the number of attributes (properties, parameters) that can be used to describe 
a thing (an "entity" or "object") is potentially very vast, but the number that will 
actually apply to a given entity is relatively modest.

= Typical Problem
A good example of this is where you need to store
lots (possible hundreds) of optional attributes on an object. My typical
reference example is when you have a User object. You want to store the
user's preferences between sessions. Every search, sort, etc in your
application you want to keep track of so when the user visits that section
of the application again you can simply restore the display to how it was.

So your controller might have:

  Project.find :all, :conditions => current_user.project_search,
    :order => current_user.project_order

But there could be hundreds of these little attributes that you really don't
want to store directly on the user object. It would make your table have too
many columns so it would be too much of a pain to deal with. Also there might
be performance problems. So instead you might do something like
this:

  class User < ActiveRecord::Base
    has_many :preferences
  end

  class Preferences < ActiveRecord::Base
    belongs_to :user
  end

Now simply give the Preference model a "name" and "value" column and you are
set..... except this is now too complicated. To retrieve a attribute you will
need to do something like:

  Project.find :all,
    :conditions => current_user.preferences.find_by_name('project_search').value,
    :order => current_user.preferences.find_by_name('project_order').value

Sure you could fix this through a few methods on your model. But what about
saving?

  current_user.preferences.create :name => 'project_search',
    :value => "lastname LIKE 'jones%'"
  current_user.preferences.create :name => 'project_order',
    :value => "name"

Again this seems to much. Again we could add some methods to our model to
make this simpler but do we want to do this on every model. NO! So instead
we use this plugin which does everything for us.

= Capabilities

The ActsAsEavModel plugin is capable of modeling this problem in a intuitive
way. Instead of having to deal with a related model you treat all attributes
(both on the model and related) as if they are all on the model. The plugin
will try to save all attributes to the model (normal ActiveRecord behavior)
but if there is no column for an attribute it will try to save it to a
related model whose purpose is to store these many sparsely populated
attributes.

The main design goals are:

* Have the eav attributes feel like normal attributes. Simple gets and sets
  will add and remove records from the related model.
* Allow for more than one related model. So for example on my User model I might
  have some eav behavior going into a contact_info table while others are
  going in a user_preferences table.
* Allow a model to determine what a valid eav attribute is for a given
  related model so our model still can generate a NoMethodError.

Example
=======

Will make the current class have eav behaviour.

  class Post < ActiveRecord::Base
    has_eav_behavior
  end
  post = Post.find_by_title 'hello world'
  puts "My post intro is: #{post.intro}"
  post.teaser = 'An awesome introduction to the blog'
  post.save

The above example should work even though "intro" and "teaser" are not
attributes on the Post model.

= Installation

    ./script/plugin install acts_as_eav_model

= RUNNING UNIT TESTS

== Creating the test database

The test databases will be created from the info specified in test/database.yml.
Either change that file to match your database or change your database to
match that file.

== Running with Rake

The easiest way to run the unit tests is through Rake. By default sqlite3
will be the database run. Just change your env variable DB to be the database
adaptor (specified in database.yml) that you want to use. The database and
permissions must already be setup but the tables will be created for you
from schema.rb.

Copyright (c) 2008 Marcus Wyatt, released under the MIT license