Rails - Feb 2007 - Inheritance design question

I''m new to ruby rails and am trying to design my first real rails web
app.

The app is intended to model a shareware business. It will provide a
place where users can learn about new projects, track their progress,
and provide feedback. Here''s a simplified list of the model objects
that
I''ll need:

Project - has title, description, creation, modification.
Release - has title, description, creation, modification belongs to
project, has tasks
Task - has title, description, creation, modification belongs to
project, belongs to release
Forum - has title, description, creation, modification has topics
Topic - has title, description, creation, modification

The key requirement is that I want these models to share some common
behavior:

- they all have a title, description, creation date, and modification
date
- i want a site wide rss feed that will include changes to Projects,
Releases, Tasks... all mixed together
- i want to maintain a full text search index accross all of these
models. So the search results will include Projects, Releases, Tasks...
- i want to do this the "right" way, and want to make it easy for
other
people to extend the system and add their own model objects that will
also share the same common behavior.

Inheritance seems like the perfect answer. I can add a page class that
all of the above models inherit from. The page class would have these
attributes and behaviors:

Page
  - has title
  - has description
  - knows how to turn self into rss feed item
  - knows how to turn self into string for text indexing

The knows how behaviors would need to be polymorphic since a subclass
might define new attributes that would also need to show up in the full
text index.

So that''s what I want, but I''m not sure what the best way to
model this
in rails is. I don''t think single-table inheritance will work in this
cases because the number of subclasses should be open ended and will
each add many of their own attributes.

It might be that (I''m getting these terms from the Agile Web
Development
with Rails book) strait inheritance using the "abstract_class" method
is
the right solution. As I understand it that would allow each concrete
class to have it''s own table. But it would require a lot of
duplication,
for example I think in my db.schema each concrete model would need to
define all the common attributes (title, description, creation,
modification) and that would be painful if I wanted to add additional
common attributes such as created_by (id of some user). I''m also not
really sure if using strait inheritance will allow me to say things like
Page.find_by_title("some title") and return a mixed list of matching
subclass instances.

The last option is to use polymorphic associations. So I would change my
Page model to look like this:

Page
  - has title
  - has description

  - knows how to turn self into rss feed item
  - knows how to turn self into string for text indexing

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> The last option is to use polymorphic associations. So I would change my
> Page model to look like this:
> 
> Page
>   - has title
>   - has description
> 
>   - knows how to turn self into rss feed item
>   - knows how to turn self into string for text indexing
Grrg... accidentally hit the entry key and my above post was posted 
before I finished...I''ll post more in a sec.

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... continuing from above.

Using polymorphic associations I could associate each page with a 
delegate Model object that the page would use to modify it''s behavior. 
Page class have one model model to look like this:

Page
  - has title
  - has description
  - has one delegate
  - delegate id
  - delegate type
  - knows how to turn self into rss feed item
  - knows how to turn self into string for text indexing

And each of the models would declare themselves as  (Project for 
example)

Project
  - delegate of page
Release
  - delegate of page
...

This approach would mean that Pages are real model objects that I''m
sure
can be searches and even uses without having a delegate set. But it also 
makes many of my models a bit more complicated since models such as 
Project, Release... will require two objects (a Page and the delegate 
model) instead of one.

Thank you very much if you''ve read this far. And please point me in the
right direction.

- what am I not thinking about that I should be thinking about?
- am i reinventing the wheel, are there other ruby rails projects that 
are already doing something similar that I could learn from?

Thank you very much for any thoughts that you can offer.

Jesse

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inheritance is overused... it should be used when the subclass is a more
specific instance of the base class in a natural way.

Ferrari < Car
Dog < Animal
Tennis < Sport

A [Project] is not naturally related to a [Page], it''s only displayed
on it
but there is no logical relationship of any kind.

In your case I would use a ruby module

module PageItem
  attr_accessor :title, :description
  def to_rss
    #
  end
  # ...
end

for example like this:

class Release < ActiveRecord::Base
  include PageItem
  belongs_to :project

  # ...
end


2007/2/6, Jesse Grosjean
<rails-mailing-list-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>:
>
>
> ... continuing from above.
>
> Using polymorphic associations I could associate each page with a
> delegate Model object that the page would use to modify it''s
behavior.
> Page class have one model model to look like this:
>
> Page
>   - has title
>   - has description
>   - has one delegate
>   - delegate id
>   - delegate type
>   - knows how to turn self into rss feed item
>   - knows how to turn self into string for text indexing
>
> And each of the models would declare themselves as  (Project for
> example)
>
> Project
>   - delegate of page
> Release
>   - delegate of page
> ...
>
> This approach would mean that Pages are real model objects that
I''m sure
> can be searches and even uses without having a delegate set. But it also
> makes many of my models a bit more complicated since models such as
> Project, Release... will require two objects (a Page and the delegate
> model) instead of one.
>
> Thank you very much if you''ve read this far. And please point me
in the
> right direction.
>
> - what am I not thinking about that I should be thinking about?
> - am i reinventing the wheel, are there other ruby rails projects that
> are already doing something similar that I could learn from?
>
> Thank you very much for any thoughts that you can offer.
>
> Jesse
>
> --
> Posted via http://www.ruby-forum.com/.
>
> >
>
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Peter Ertl wrote:
> inheritance is overused... it should be used when the subclass is a more
> specific instance of the base class in a natural way.
> 
> Ferrari < Car
> Dog < Animal
> Tennis < Sport
> 
> A [Project] is not naturally related to a [Page], it''s only
displayed on
> it
> but there is no logical relationship of any kind.
Peter,

Thanks... I really do know this stuff, but in all the confusion of a new 
language and programming model I got subclass happy. Thanks for getting 
me back on track. In your example what would the final database schema 
look like?

  create_table "project", :force => true do |t|
    t.column "title", :string
    t.column "description", :text
  end

or more like:

  create_table "project", :force => true do |t|
  end

  create_table "page_item", :force => true do |t|
    t.column "tagged_id", :integer
    t.column "tagged_type", :string
    t.column "title", :string
    t.column "description", :text
  end

Am I correct that it could be implemented either way?

In this case it seems to make the most sense to just store the data in 
the project table since title and description are important parts of the 
project.

But on the other hand that means that I''ll have a lot of model objects 
that are all declaring the same basic set of attributes over and over 
again (title, description, created_at...). I could get rid of that by 
storing things in the database the second way, but it''s a bit more 
complex and slower I would imagine.

So are their any suggestions or rules of thumb to help with this kind of 
decision?

Thanks,
Jesse

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