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 yourGemfile
- Run
rails generate updateable_views_inheritance:install && rake db:migrate
- In
config/environment.rb
setconfig.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
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 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.
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.