Rails - Sep 2006 - Need help understanding self.abstract_class = true in AR

All,

I submitted the following bug to Rails Traq: 
http://dev.rubyonrails.org/ticket/6010

I was basing my bug report on my understanding that setting 
self.abstract_class = true meant that your model did not have a table 
backing it.  This has been my experience when using the odbc adapter for 
AR against SQL Server, where I have created several abstract models and 
instantiated them successfully without having backing tables for them.

However, the traq report says that self.abstract_class = true "means you 
can inherit from it (the model) without using STI."

The RoR API says: "Returns whether this class is a base AR class. If A 
is a base class and B descends from A, then B.base_class will return B."

Q1) Can someone please summarize what the purpose of setting 
self.abstract_class is?

Q2) If self.abstract_class = true is NOT meant to create an ActiveRecord 
model that is not backed by a table, then what is the correct way to 
accomplish that?

Neither of those statements above addresses whether or not the 
abstract_class is expected to have a backing table or not.  To me, it''s
implied that there is not a backing table (I don''t see how you could 
even take advantage of one if there were a backing table).

It makes sense that perhaps it is not expected that an abstract AR class 
would even be instantiated so handling issues with around the existence 
of a backing table wouldn''t be addressed.

Thanks,
Wes

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Wes Gamble wrote:
> Q2) If self.abstract_class = true is NOT meant to create an ActiveRecord
> model that is not backed by a table, then what is the correct way to
> accomplish that?
I''m a bit flummoxed by why you''d want an ActiveRecord object
that
didn''t have a database table behind it. What would it do? The whole
point of ActiveRecord is to map the object to the database.

If you just want a class that isn''t saved to the database, you can
create a normal Ruby class like

class Person
end

and put it in your /app/models directory and use it.

Chris


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I want to take advantage of AR validation methods for such a model.


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I use an abstract class in my FlexImage plugin 
http://beautifulpixel.com/articles/2006/08/21/fleximage

Wes Gamble wrote:
> Q1) Can someone please summarize what the purpose of setting 
> self.abstract_class is?
Basically, I wanted to provide a class that one of your models could 
inherit from with all the functionality.  The problem was that Rails was 
assuming I wanted to do STI when I just wanted basic ruby inheritance. 
Setting abstract class in this case meant there was no table and it only 
exists as a class to inherit from and provides no functionality in and 
of itself.

This is a longer version of what the reply in that ticket says.  It''s 
absolutely right.  Abstract AR classes aren''t used idrectly, just 
inherited from.
> Q2) If self.abstract_class = true is NOT meant to create an ActiveRecord 
> model that is not backed by a table, then what is the correct way to 
> accomplish that?
If you want an ActiveRecord object without a DB table, then you don''t 
want an ActiveRecord object.  The entire core functionality of AR is 
built around providing an interface to a database table.

If you want a class to validate other AR objects, abstract class will 
work for you.

class Foo < ActiveRecord::Base
  self.abstract_class = true
  validates_presence_of :bar
end

class Zing < Foo
  # already inherits validates_presence_of :bar
end

class Groucho < Foo
  # already inherits validates_presence_of :bar
end

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Alex,

Thanks for the response.

So if I can actually create an instance of an AR object that has 
self.abstract_class set to true, then that should be a bug, correct?

I am able to create instances of abstract AR classes when using the ODBC 
driver against SQL Server.  Should I log that as a bug?

I still would like to be able to get the AR::Validations functionality 
for an arbitrary class - I''ll play with seeing if I can just include it
into a regular class.

Thanks,
Wes

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On 9/1/06, Wes Gamble
<rails-mailing-list-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
wrote:
>
> All,
>
> I submitted the following bug to Rails Traq:
> http://dev.rubyonrails.org/ticket/6010
>
> I was basing my bug report on my understanding that setting
> self.abstract_class = true meant that your model did not have a table
> backing it.  This has been my experience when using the odbc adapter for
> AR against SQL Server, where I have created several abstract models and
> instantiated them successfully without having backing tables for them.
>
> However, the traq report says that self.abstract_class = true "means
you
> can inherit from it (the model) without using STI."
>
> The RoR API says: "Returns whether this class is a base AR class. If A
> is a base class and B descends from A, then B.base_class will return
B."
>
> Q1) Can someone please summarize what the purpose of setting
> self.abstract_class is?
>
> Q2) If self.abstract_class = true is NOT meant to create an ActiveRecord
> model that is not backed by a table, then what is the correct way to
> accomplish that?
1) read my response to the ticket:

An abstract model means you can inherit from it without using STI.

class SuperModel < AR::Base
  self.abstract_class = true
end

class Foo < SuperModel
end

Foo.table_name
# => ''foos''

It''s used in the CachedModel plugin.  I''m not sure why
it''s a class
and not a mixin though, but ok.

2)  Two ideas, one is really hackish.

The hackish way:

http://rails.techno-weenie.net/tip/2005/11/19/validate_your_forms_with_a_table_less_model

The second is directly including ActiveRecord::Validations into your
class.  You may have to implement some of the ActiveRecord API for
this though.  I''m pretty sure the wiki has some info on this.


-- 
Rick Olson
http://weblog.techno-weenie.net
http://mephistoblog.com

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Rick,

I get it now.  However, my original issue has to do with what I now 
believe to be a bug in the AR ODBC module.

Using AR w/ODBC against SQL Server, I can successfully instantiate a 
class with self.abstract_class set to true with no problems.  I 
convinced myself that part of the intended functionality of 
self.abstract_class was to allow the user to take advantage of AR 
validations without requiring them to persist the data.

Now I see that that was an incorrect understanding.

Which now makes me realize that the original behavior I see against SQL 
Server is the bug.  Do you think that''s a bug?

Wes

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On 02/09/06, Wes Gamble
<rails-mailing-list-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org> wrote:
> Which now makes me realize that the original behavior I see against SQL
> Server is the bug.  Do you think that''s a bug?
As Rick said, setting self.abstract_class = true means you can extend
a class without STI kicking in.  The class isn''t really abstract in
the strict sense; it can still be instantiated (whichever database
adapter is used).  Perhaps base_class or some other signifier
(acts_as_base_class?) would have been less confusing.

Tom

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So I think there''s a bug in the ODBC mode of the SQL Server driver
then.

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Rick Olson wrote:
> The second is directly including ActiveRecord::Validations into your
> class.  You may have to implement some of the ActiveRecord API for
> this though.  I''m pretty sure the wiki has some info on this.
Here''s a nice article about a plugin that looks like it should do the 
trick:
http://www.realityforge.org/articles/2005/12/02/validations-for-non-activerecord-model-objects

Wes

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This plugin seems to work quite well.


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