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.

Requirements

Rails: 4.x

Ruby: 1.9.3+

Database: PostgreSQL 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.

Usage

Setup

• Add gem 'updateable_views_inheritance' to your Gemfile
• Run rails generate updateable_views_inheritance:install && rake db:migrate
• In config/environment.rb set config.active_record.schema_format = :sql

Example

The database migration:

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

In the parent

class ChangeColumnsInParentTable < 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

In a child

class ChangeColumnInChildTable < ActiveRecord::Migration
  def self.up
    drop_view(:steam_locomotives)
    rename_column(:steam_locomotives_data, :coal_consumption, :fuel_consumption)
    create_child_view(:locomotives, :steam_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 drop_child(child_view) in migrations.

Using parent class without instantiating subclass

If you don't want to make a second SQL query to the subclass table when you instantiate parent class with Locomotive.find(1) use

class Locomotive
  self.disable_inheritance_instantiation = true
end

Quite handy for flat and wide class hierarchies (one parent class, many subclasses).

Using existing table for inherited class

class CreateIkarusBus < ActiveRecord::Migration
  def self.up
    # table `tbl_ikarus_buses` exists in the database
    end
    create_child(:ikarus_buses,
                 table: :tbl_ikarus_buses,
                 parent: :buses,
                 skip_creating_child_table: true)
  end
end

Useful when converting legacy DB schema to use inheritance.

Using view as a child table

      execute <<-SQL.squish
        CREATE VIEW punk_locomotives_data AS (
          SELECT steam_locomotives.id,
                 steam_locomotives.coal_consumption AS coal,
                 NULL AS electro
          FROM steam_locomotives
          UNION ALL
          SELECT electric_locomotives.id,
                 NULL AS coal,
                 electric_locomotives.electricity_consumption AS electro
          FROM electric_locomotives)
      SQL
      create_child(:punk_locomotives,
                   { parent: :locomotives,
                     child_table: :punk_locomotives_data,
                     child_table_pk: :id,
                     skip_creating_child_table: true })

Views in PostgreSQL cannot have primary keys, so you have to manually specify it when you use. Note that views also cannot have NOT NULL constraints, although the NOT NULL constraint of the underlying table will still be enforced.

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 rake updateable_views_inheritance:fixture to generate fixture for the updateable_views_inheritance table after you add/remove classes from the hierarchy or change underlying table or view names. Without it primary key sequence for inheritors' tables won't be bumped to the max and it might not be possible to save objects! 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 test_helper.rb:

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

for all fixtures you want to load.

Gem Development & Testing

In order to run gem tests, you have to be a superuser in PostgreSQL.