With the attr_inherited gem, your ActiveRecord models can inherit any of their 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 inherit title and synopsis from the associated post, when the attributes are nil

post = Post.create!(title: "First post", synopsis: "First post synopsis")
version = Version.create!(post: post)

version.title
 => "First post"

version.synopsis
 => "First post synopsis"

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

version.update_attributes(synopsis: "Version synopsis")

version.synopsis
 => "Version synopsis"

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

post = Post.create!(synopsis: "First post synopsis")
version = Version.create!(post: post)

version.description
 => "First post synopsis"

version.update_attributes(description: "Version description")

version.description
 => "Version description"

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

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

post = Post.create!(title: "First post")
version = Version.create!(post: post)

version.title
 => "First post"

version.update_attributes(title: "     ")
version.title
 => "First post"

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:

post = Post.create!(title: "First post")
post.comments.create(text: "Post comment")

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

version.comments = [Comment.create(text: "Version comment")]
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

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

1.0.0 Initial relase (2012-10-02)

• Implement _inherited?

  def #{_alias}_inherited? #{_parent}.#{attr} == self.#{attr} end

• Test assertion of valid parameters

• Make 1.8.7/ree/rbx compatible

• Write rdocs

• Document or remove default parent option for from: option

• Properly support inheriting associations

• Investigate support for virtual attributes (:attr_accessor)

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

Copyright (c) Christos Zisopoulos, released under the MIT license