File: README — Documentation for attr

Description

Attribute inheritance for ActiveRecord models

Using attr_inherited allows your ActiveRecord models to inherit any of their attributes from an associated, or parent model.

Usage

Inheriting attributes from an associated model

Given a Post model and the following Version model

class Version < ActiveRecord::Base
  belongs_to  :post

  attr_inherited :title, :synopsis, from: :post
end

..then Version will now inherit title and synopsis from the parent post, if either attributes are nil

1.9.3p194 :001 > post = Post.create!(title: "First post", synopsis: "First post synopsis")
1.9.3p194 :002 > version = Version.create!(post: post)

1.9.3p194 :003 > version.title
 => "First post"

1.9.3p194 :004 > version.body
 => "First post body"

You can override any of the inherited attributes by setting its a value to anything other than nil

1.9.3p194 :005 > version.update_attributes(description: "Version synopsis")

1.9.3p194 :006 > version.synopsis
 => "Version description"

Renaming inherited attributes

If you want the inherited attribute to have a name other than that of the associated model's attribute, you can use the as: option to rename it. For example:

class Version < ActiveRecord::Base
  belongs_to  :post

  attr_inherited :synopsis, from: :post, as: :description
end

1.9.3p194 :001 > post = Post.create!(synopsis: "First post synopsis")
1.9.3p194 :002 > version = Version.create!(post: post)

1.9.3p194 :003 > version.description
 => "First post synopsis"

Note that description must be an attribute of Version for the above to work.

Inheriting attributes even when they are not `nil`

You might want to inherit an attribute not only when it is nil but even when it is an empty string "". You can do this by passing a predicate with the when: option that would test if the attribute should be inherited.

class Version < ActiveRecord::Base
  belongs_to  :post

  attr_inherited :title, from: :post, when: :blank?
end

1.9.3p194 :001 > post = Post.create!(title: "First post")
1.9.3p194 :002 > version = Version.create!(post: post)

1.9.3p194 :003 > version.title
 => "First post"

1.9.3p194 :004 > version.update_attributes(title: "     ")
1.9.3p194 :005 > version.title
 => "First post"

Using the `when:` option to fake has_many association inheritance

Until real association inheritance is implemented you can fake it like this:

class Version < ActiveRecord::Base
  has_many :comments, as: :commentable
end

class Version < ActiveRecord::Base
  belongs_to  :post
  has_many :comments, as: :commentable

  attr_inherited :comments, from: :post, when: :empty?
end

...which makes comments behave almost as if it was inherited from Post:

1.9.3p194 :001 > post = Post.create!(title: "First post")
1.9.3p194 :002 > post.comments.create(text: "Post comment")

1.9.3p194 :003 > version = Version.create!(post: post)
1.9.3p194 :004 > version.comments.first.text
 => "Post comment"

1.9.3p194 :005 > version.comments = [Comment.create(text: "Version comment")]
1.9.3p194 :006 > version.comments.first.text
 => "Version comment"

Caveat: You can't call any other association methods on version.comments when it is inherited, such as <<, .create, .build

Installation

Requires Rails 3.1+

Add this line to your application's Gemfile:

gem 'attr_inherited'

And then execute:

$ bundle

Or install it yourself as:

$ gem install attr_inherited

Changelog

1.0.0 Initial relase (2012-10-02)

To Do

Test assertion of valid parameters
Write rdocs
Document or remove default parent option for from: option
Properly support inheriting associations
Investigate support for virtual attributes (:attr_accessor)

Contributing

Fork it
Create your feature branch (git checkout -b my-new-feature)
Commit your changes (git commit -am 'Add some feature')
Push to the branch (git push origin my-new-feature)
Create new Pull Request