HasCustomFields

HasCustomFields 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 HasCustomFields 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_custom_field_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_custom_field_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.