Heritage
Heritage is a gem that implements Multiple Table Inheritance for ActiveRecord models.
Compatability
Heritage has only been tested with Rails 3
Installation
Simply add Heritage to your Gemfile and bundle it up:
gem 'heritage'
Usage
Heritage works by assigning one model as your predecessor, and one or more other models as it’s heir.
The predecessor is the parent of it’s heirs, and thereby implicitly gives it’s heirs access to it’s columns, and optionally exposing methods to them.
To mark a model as predecessor, simply use the acts_as_predecessor class-method:
class Post < ActiveRecord::Base
acts_as_predecessor
end
To mark a model as heir, simply use the acts_as_heir_of class-method, passing a symbol to the model that is to be the heirs predecessor.
class BlogPost < ActiveRecord::Base
acts_as_heir_of :post
end
This takes care of the model configuration. We however need to add two extra columns to the Posts table.
We need a heir_id column of type integer and a heir_type column of type string.
class CreatePosts < ActiveRecord::Migration
def self.up
create_table :posts do |t|
t.integer :heir_id
t.string :heir_type
t.string :title
t.
end
end
def self.down
drop_table :posts
end
end
class CreateBlogPosts < ActiveRecord::Migration
def self.up
create_table :blog_posts do |t|
t.text :body
end
end
def self.down
drop_table :blog_posts
end
end
When this is done and the database is migrated, we can begin using the models.
Creating new instances
Now we can simply call the following to create a new BlogPost
blog_post = BlogPost.create(:title => "Wow", :body => "That's a nice blog post!")
Notice that the title attribute belongs to the Post model, and the body attribute belongs to the BlogPost model.
Attributes
We can directly access the title attribute through BlogPost and even change it’s value
blog_post.title # "Wow"
blog_post.title = "Oh boy!"
blog_post.save!
blog_post.title # "Oh boy!"
We can also update attributes like normal through update_attributes
blog_post.update_attributes(:title => "Hubba Hubba", :body => "Nice blog post!")
blog_post.title # "Hubba Hubba"
blog_post.body # "Nice blog post!"
Methods
If we want to expose some methods from our predecessor model to it’s heirs, we can do so when calling the acts_as_predecessor class-method
class Post < ActiveRecord::Base
acts_as_predecessor :exposes => :hello
def hello
"Hi there!"
end
end
Now all heirs of Post will have a hello-method, which we can call directly on the heir-model:
blog_post = BlogPost.create(:title => "I am full", :body => "of methods...")
blog_post.hello # "Hi there!"
If you for some reason need to override the method in one of your heir-models, you can simply implement the method, and it will override the method from the predecessor.
class BlogPost < ActiveRecord::Base
acts_as_heir_of :post
def hello
"Yo!"
end
end
Calling the hello method on BlogPost will now yield another result:
blog_post = BlogPost.create(:title => "I have", :body => "my own methods...")
blog_post.hello # "Yo!"
If we need to combine the local method in the heir, with the method in the predecessor, we can do so through the predecessor method of the heir model, kinda like you would use super.
class BlogPost < ActiveRecord::Base
acts_as_heir_of :post
def hello
"Yo! #{predecessor.hello}"
end
end
The result would now be a combination of the local method in the heir, and the method in the predecessor:
blog_post = BlogPost.create(:title => "I have", :body => "my own methods...")
blog_post.hello # "Yo! Hi there!"
Listing and filtering
To list all your wonderful heir models you do as you normally would in ActiveRecord, with one single exception.
Normally you would call something like this, to show all BlogPosts
@posts = BlogPost.all
This however will result in 1 + the number of returned records SQL calls, which is hardly good. Instead you need to tell ActiveRecord that it should include the predecessors of the heirs, like so:
@posts = BlogPost.all(:include => :predecessor)
We now only call the database twice; Once for loading the heirs, and once for loading all referenced predecessors.
Another gotcha is when you need to filter the heirs. You can’t directly filter by attributes from the predecessor model.
So in our example where we have the title attribute in the Post model, we can’t do the following:
@posts = BLogPost.where("title = 'test'")
Instead we need to reference predecessor attributes by the predecessors database-table, like so:
@posts = BlogPost.where("posts.title = 'test'")
Behind the scenes, heritage works just like a simple ActiveRecord association, so it makes sense.
Timestamps
If all of your heir-models needs timestamps, then you can simply add timestamps to the predecessor model, and omit them from the heir-models.
Heritage will make sure, that whenever you update your heir-model, the updated_at timestamp in the predecessor model will be updated.
A note on destruction
Heritage depends on the destroy-method of the models, and as such you should always delete predecessor and heir models by calling the destroy method on either, and NEVER by calling the delete or delete_all methods.
If you absolutely need to do a direct delete in the database, then you need to manually remove the counterpart as well.
For instance, if you manually delete a BlogPost that is heir of Post, then you need to first find the right Post, then delete the heir and finally delete the predecessor.
Advanced usage
It is always possible to traverse between a predecessor and it’s associated heir, through the predecessor method of an heir, and the heir method of a predecessor.
Questions, Feedback
Feel free to message me on Github (murui)
Contributing to Heritage
Fork, fix, then send me a pull request.
Credits
Credits goes out to Gerry from TechSpry.com for the idea for this implementation: http://techspry.com/ruby_and_rails/multiple-table-inheritance-in-rails-3/
License
Heritage by Thomas Dippel @ Benjamin Media A/S is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.
Based on a work at techspry.com