Rails - Aug 2005 - Single table inheritance

Does anyone has experience implementing single table inheritance.  Does it
really work?

This is my situation,  I trying to have a base table call category.  then I
have a model call TodoCategory to inherit category,  HelpDeskCategory to
inherit category.

After that I have TodoItem model that have a field call TodoCategory_id,
the story goes on.  I am having a really hard time to get it work.

Dose this single table inheritance really works? or do I miss out something
here.

Paul,


This is my situation, I trying to have a base table call category. then
I
> have a model call TodoCategory to inherit category, HelpDeskCategory to
> inherit category.

I''m using it almost the exact same way. I have a User table and 3
models
that associate with it: User, Caregiver and Admin.

class User < ActiveRecord::Base

class Admin < User

class CareGiver < User

Now, the one thing that gave me some problems were the requires. I
don''t
know if they''re still relevant or not, but I had to put require
''admin'' and
require ''caregiver'' lines in the User model.

After that I have TodoItem model that have a field call
TodoCategory_id,
> the story goes on. I am having a really hard time to get it work.
> 
The way that Rails handles STI is via a type column in the table that is 
populated with the type of object you want that record to reference. So, in 
your case, maybe a little something a-like this:

class Category < ActiveRecord::Base

class TodoCategory < Category

class HelpDeskCategory < Category

And in your Category table, remove the TodoCategory_id column and replace it 
with type. For every ToDoCategory row, type must contain TodoCategory. The 
same holds for HelpDeskCategory.

Hope that works, or at least points you in the right direction.

--
Mando


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Interesting.

This introduces a nasty redundancy.

The way I have always handled STI is via something such as:


people
-----------
id
first_name
last_name

users
---------
person_id
email
active

user_passwords
-----------------------
user_id
password



I haven''t yet implemented STI in Rails, but am currently trying to  
decide whether it fits a very robust publishing tool I''ve had  
designed for some time now and have partially implemented in other  
languages.

I imagine the model classes might look something like:

class Person < ActiveRecord::Base

end

class User < Person
     set_primary_key "person_id"
end


I imagine this means I''ll have to manually join the tables, which I  
suppose is fine. It''s far better than redundancy, IMO, and gives me a  
chance to optimize my queries for my specific RDBMS. It is  
unfortunate if the "type" column approach is the only way STI will be
supported in Rails, though.

-tj





On Aug 5, 2005, at 9:19 AM, Mando Escamilla wrote:
>  For every ToDoCategory row, type must contain TodoCategory.  The  
> same holds for HelpDeskCategory.

Toby,
> The way I have always handled STI is via something such as:
> 
> people
> -----------
> id
> first_name
> last_name
> 
> users
> ---------
> person_id
> email
> active
Actually, this isn''t Single Table Inheritance at all... you are
describing a different O-R mapping pattern.

There are several different patterns for mapping the inheritance
hierarchy of classes in an object oriented language onto a relational
database.  ActiveRecord uses Single Table Inheritance.  What you
describe in your example is closer to the Class Table Inheritance
pattern.  There are advantages and disadvantages of both.  For more
info, look at the relevant sections in chapter 12 of _Patterns of
Enterprise Application Architecture_ by Martin Fowler.

Ken

You''re right -- I used the wrong name/concept. Woops!

I have and know the Fowler book, but mixed up my ideas when writing  
on my commute. :(

On Aug 5, 2005, at 1:38 PM, Ken Kunz wrote:
> Toby,
>
>
>> The way I have always handled STI is via something such as:
>>
>> people
>> -----------
>> id
>> first_name
>> last_name
>>
>> users
>> ---------
>> person_id
>> email
>> active
>>
>
> Actually, this isn''t Single Table Inheritance at all... you are
> describing a different O-R mapping pattern.
>
> There are several different patterns for mapping the inheritance
> hierarchy of classes in an object oriented language onto a relational
> database.  ActiveRecord uses Single Table Inheritance.  What you
> describe in your example is closer to the Class Table Inheritance
> pattern.  There are advantages and disadvantages of both.  For more
> info, look at the relevant sections in chapter 12 of _Patterns of
> Enterprise Application Architecture_ by Martin Fowler.
>
> Ken
> _______________________________________________
> Rails mailing list
> Rails-1W37MKcQCpIf0INCOvqR/iCwEArCW2h5@public.gmane.org
> http://lists.rubyonrails.org/mailman/listinfo/rails
>

I am using STI to model organizations (department < organization, 
company < organization) so I can use Composite Pattern.
I am using migrations to do the DDL for MySql

class CreateCompany < ActiveRecord::Migration
  def self.up
    create_table :organizations do |t|
      t.column :name, :string
      t.column :label, :string
      t.column :phone, :string
      t.column :fax, :string
      t.column :address, :string
      t.column :type,     :string
      t.column :position, :integer
      t.column :parent_id, :integer
      t.column :costcode, :string
    end
  end

  def self.down
    drop_table :organizations
  end
end
------------
here''s my organization.rb file
class Organization < ActiveRecord::Base
  validates_presence_of :name
end

class Company < Organization
  has_many :departments
end

class Department < Organization

  before_save :copy_parent

  acts_as_tree
 belongs_to :company, :foreign_key => ''company_id''
  validates_presence_of :company_id

  def copy_parent
     self.type = self.parent.type if self.parent
    self.company = self.parent.company if self.parent
  end
end

class FaaDepartment < Department
 validates_presence_of :costcode
  validates_uniqueness_of :costcode
end
------------------------

What I want to happen is if a company has a root department, every 
department that is created from that root with the :children method
inherits the same company and type of the parent department

company = Company.new(:name => ''FAA'')
company.save
root = company.departments.create(:name => ''ANI'', :type
=> ''FaaDepartment'')

now I want root.children.create(:name => ''ANI-2'') to
inherit the
company_id and the type from its parent but copy_parent from above 
doesn''t work.

Any suggestions?

Thanks in advance,
dom