EmFi/mixed_table_inheritance

Ruby on Rails plugin to get the benefits from Single Table Inheritance in cases where subclasses with unique fields make STI infeasible.

Mixed Table Inheritance
=======================

Mixed Table Inheritance(MTI) mitigates some of the drawbacks of Single Table
Inheritance(STI). Providing an elegant solution for cases where models share
common elements but have enough unique elements to make STI infeasible.

MTI uses STI to store common fields and functionality of related models and
offloads unique elements to another table. While seamlessly accessing both the
common fields and functionality inherited through STI and the unique fields and
functionality of the sub model.

Usage
=====

To add Mixed Table Inheritance to a sub model just add the following line anywhere
in the body of the inheriting model.

  mti_subclass

Mixed Table Inheritance uses Single Table Inheritance to store fields,
associations, validations and other model logic common to your models. The specific
fields for each submodel are stored in separate table for each submodel. Which means that
Mixed Table Inheritance requires an additional table and model to operate.
A generator is provided to create those models and migrations.

$ script/generate mti_model <SubClassName> <ParentClassName> [attribute:type, attribute:type, ...]

Where SubClassName is the name of inheriting class, ParentClassName is the name of the inherited
class, either class name can be supplied in CamelCase or underscore_case. attribute:type follows
the same syntax of script/generate model, and details the fields specific to SubClassName.

If SubClassName doesn't exist it will be created and populated with mti_subclass.
A migration creating a sub_class_name_details table and a corresponding model are
also created by the migration.


Example
=======

Take the relationship between the following models:

Employee (fields: id, name, address, start date)
SalariedEmployee (fields: id, name, address, start date, salary)
ContractEmployee (fields: id, name, address, start date, salary, end date)
HourlyEmployee (fields: id, name, address, start date, hours, rate)

It is tempting to use SingleTableInheritance for this relationship, because of
all the common fields and logic that can be put into the Employee model. However
there are fields that will be never be used together. Mixed Table Inheritance
gives you the advantages of Single Table Inheritance such as the guarantee that
all decendents have access to the same fields and methods. While at the same time
greatly cuts down on the wasted space in the database.

To invoke mixed table inheritance add mti_subclass to your inheriting models.

class Employee < ActiveRecord::Base
  #common associations/validations/etc
end

class SalariedEmployee < Employee
  mti_sublcass
  # associations/validations/etc specific to SalariedEmployee
  # ...
end

class ContractEmployee < Employee
  mti_sublcass
  # associations/validations/etc specific to ContractEmployee
  # ...
end

class HourlyEmployee < Employee
  mti_sublcass
  # associations/validations/etc specific to HourlyEmployee
  #  ...
end

$ script/generate mti_model SalariedEmployee Employee salary:integer
$ script/generate mti_model ContractEmployee Employee salary:integer, end_date:date
$ script/generate mti_model HourlyEmployee Employee rate:float, hours:string

To Do
======
Documentation
Tests

Copyright (c) 2010 [name of plugin creator], released under the MIT license