==Class Table Inheritance

Class Table Inheritance for ActiveRecord using updateable views.

More about the pattern on http://www.martinfowler.com/eaaCatalog/classTableInheritance.html. This gem messes very little with Rails inheritance mechanism.
Instead it relies on updatable views in the database to represent classes in the inheritance chain. The approach was {first suggested by John
Wilger}[http://web.archive.org/web/20060408145717/johnwilger.com/articles/2005/09/29/class-table-inheritance-in-rails-with-postgresql].


==Requirements

Rails: 3.2.x

Ruby: tested with 1.8.7 and 1.9.3

Database: PostgreSQL 8.1+ only. Patches for other DBMS are welcome. Note that you are not required to use updateable views, children relations can be tables (with some triggers involved) or materialized views.

==Install
Run
  gem install updateable_views_inheritance

==Usage

===Setup

* In <tt>Gemfile</tt> add <tt>gem 'updateable_views_inheritance'</tt>
* Run <tt>rails generate updateable_views_inheritance:install && rake db:migrate</tt>
* In <tt>config/environment.rb</tt> set <tt>config.active_record.schema_format = :sql</tt>
* In case you're using fixtures, don't forget to run

    rake updateable_views_inheritance:fixture

after every change to the class hierarchy. Otherwise tests may fail.

===Example

  class CtiExample < ActiveRecord::Migration
    def self.up
      create_table :locomotives do |t|
        t.column :name, :string
        t.column :max_speed, :integer
        t.column :type, :string
      end

      create_child(:steam_locomotives, :parent => :locomotives) do |t|
        t.decimal :water_consumption, :precision => 6, :scale => 2
        t.decimal :coal_consumption,  :precision => 6, :scale => 2
      end

      create_child(:electric_locomotives, :table => :raw_electric_locomotives, :parent => :locomotives)  do |t|
        t.decimal :electricity_consumption, :precision => 6, :scale => 2
      end
    end

    def self.down
      drop_child  :steam_locomotives
      drop_child  :electric_locomotives
      drop_table  :locomotives
    end
  end

And the models:
  class Locomotive
  end

  class SteamLocomotive < Locomotive
    self.table_name =  :steam_locomotives
  end

  class ElectricLocomotive < Locomotive
    self.table_name =  :electric_locomotives
  end

Note that models of children classes must specify table name explicitly.

===Changing Columns in Underlying Tables

  class RemoveColumnInParentTable < ActiveRecord::Migration
    def self.up
      remove_parent_and_children_views(:locomotives)
      remove_column(:locomotives, :max_speed)
      rename_column(:name, :title)
      rebuild_parent_and_children_views(:locomotives)
    end
  end

===Renaming Underlying Tables

  remove_parent_and_children_views(:old_name)
  rename_table(:old_name,:new_name)
  execute "UPDATE updateable_views_inheritance SET child_aggregate_view = 'new_name' WHERE child_aggregate_view = 'old_name'"
  execute "UPDATE updateable_views_inheritance SET parent_relation = 'new_name' WHERE parent_relation = 'old_name'"
  rebuild_parent_and_children_views(:new_name)

===Removing Classes

Note that you should remove only leaf classes (i.e. those that do not have descendants). If you want to erase a whole chain or part of chain you have to remove first the leaves and then their ancestors. Use <tt>drop_child(child_view)</tt> in migrations.

==Compatibility with Single Table Inheritance

The approach of this gem is completely independent from Rails built-in Single Table Inheritance. STI and CLTI can safely be mixed in one inheritance chain.

==Testing Your App

If you use fixtures, you must run <tt>rake updateable_views_inheritance:fixture</tt> to generate fixture for the updateable_views_inheritance table after you add/remove
classes from the hierarchy or change underlying table or view names. <b>Without it primary key sequence for inheritors' tables won't be bumped to the max and it might not be possible to save objects!</b> If you don't use fixtures for the classes in the hierarchy you don't need to do that.

This gem re-enables referential integrity on fixture loading. This means that

  fixtures :all

may fail when there are foreign key constraints on tables. To fix this, explicitly declare fixture load order in <tt>test_helper.rb</tt>:

  fixtures :roots, :trunks, :leafs, ...

for all fixtures you want to load.

==Gem Development & Testing

The gem has a comprehensive test suite. In order to run it, your user must be a superuser in PostgreSQL.
If this is not the case, run <tt>createuser -s pesho</tt> (assuming your Unix account is <tt>pesho</tt>).