Rails - Feb 2006 - Models and Inheritance

I have a table of companies and a join table forming the relationship 
between a list of possible services they can provide.

Working from a basic understanding of OOP I assumed that there should be 
a series of Serviceprovider child objects of the company class.

Company < ActiveRecord::Base
  has_belongs_to_many :services
end

Serviceprovidertype1 < Company

Serviceprovidertype2 < Company

These child classes have corresponding child controllers

Serviceprovidertype1Controller < CompanyController

in the parent I have a method

protected
list_by_service(service)
  Company.find_by_sql(.. + service + ...)
end

and in the child Serviceprovidertype1Controller

list
  list_by_service(''Servicetype'')
end

but it only seems to work if I make the method in the parent a class 
method ie

Company.list_by_service(service)

I don''t understand why the child just doesn''t inherit the
parent method,

and why a class method should  - I''ve read it''s used  when you
want it
to apply the same to all object instances ie

count_the_number of companies

Can anyone explain what I''ve obviously misunderstook, is my appraoch 
using inheritance in this manner inappropriate ?

_tony






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> and in the child Serviceprovidertype1Controller
> 
> list
>   list_by_service(''Servicetype'')
> end
> 
> but it only seems to work if I make the method in the parent a class 
> method ie
> 
> Company.list_by_service(service)

I *think* my problem was that I *was* calling the parent method as if it 
was a class method

list
  Comapny.list_by_service(''Servicetype'')


Anyways I started reading about the how a ''type'' column can be
used with
child classes but only with has_one, belongs_to relationship.

Is there similar work on objects with HABTM relationships, as in my 
example where a company has many services and saving the child model 
updates the company details and the service details ?

_Tony

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