h1. Heritage
Heritage is a gem that implements Multiple Table Inheritance for ActiveRecord models.
h2. Compatability
Heritage has only been tested with Rails 3
h2. Installation
Simply add Heritage to your Gemfile and bundle it up:
gem 'heritage'
h2. 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.timestamps 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.
h2. 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.
h2. 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!"
h2. 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!"
h2. 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 join the predecessor attributes by its association, like so:
@posts = BlogPost.joins(:predecessor).where("posts.title = 'test'")
Behind the scenes, heritage works just like a simple ActiveRecord association, so it makes sense.
h2. 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.
h2. 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.
h2. 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.
h2. Questions, Feedback
Feel free to message me on Github (murui)
h2. Contributing to Heritage
Fork, fix, then send me a pull request.
h2. 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/
h2. 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