Rails - Aug 2005 - Defining classes/objects as read-only?

Is there an elegant way to make all instances of a class derived from 
AchtiveRecord read-only? It must be possible to load objects from the 
DB, of course, but after that these objects should be immutable.

Michael

-- 
Michael Schuerig                       Face reality and stare it down
mailto:michael-q5aiKMLteq4b1SvskN2V4Q@public.gmane.org        --Jethro Tull,
Silver River Turning
http://www.schuerig.de/michael/

Michael Schuerig wrote:
>Is there an elegant way to make all instances of a class derived from 
>AchtiveRecord read-only? It must be possible to load objects from the 
>DB, of course, but after that these objects should be immutable.
>
>Michael
>
>  
>
I''m fairly new to Ruby, but would freezing the instance work?

If so, then you could have all your classes inherit from a "frozen" 
class that inherits form ActiveRecord::Base.  In your frozen class you 
could overrid the find() or instantiate() methods (from base.rb) and 
call super() then freezing the returned rows yourself.

class Frozen < ActiveRecord::Base

  def instantiate(*args)
    object = super
    object.freeze
  end

end

On Wednesday 17 August 2005 17:16, Steve Downey wrote:
> Michael Schuerig wrote:
> >Is there an elegant way to make all instances of a class derived
> > from AchtiveRecord read-only? It must be possible to load objects
> > from the DB, of course, but after that these objects should be
> > immutable.
> I''m fairly new to Ruby, but would freezing the instance work?
No, apparently it does not. As I don''t know how freezing works exactly,
I don''t know why.

Michael

-- 
Michael Schuerig                       All good people read good books
mailto:michael-q5aiKMLteq4b1SvskN2V4Q@public.gmane.org                Now your
conscience is clear
http://www.schuerig.de/michael/ --Tanita Tikaram, Twist In My Sobriety

Does Rails not override the freeze method for it''s own purposes?
Perhaps it aliases the original version of freeze when it does so?

David

On 8/17/05, Michael Schuerig
<michael-q5aiKMLteq4b1SvskN2V4Q@public.gmane.org>
wrote:
> On Wednesday 17 August 2005 17:16, Steve Downey wrote:
> > Michael Schuerig wrote:
> > >Is there an elegant way to make all instances of a class derived
> > > from AchtiveRecord read-only? It must be possible to load objects
> > > from the DB, of course, but after that these objects should be
> > > immutable.
> 
> > I''m fairly new to Ruby, but would freezing the instance work?
> 
> No, apparently it does not. As I don''t know how freezing works
exactly,
> I don''t know why.
> 
> Michael
> 
> --
> Michael Schuerig                       All good people read good books
> mailto:michael-q5aiKMLteq4b1SvskN2V4Q@public.gmane.org                Now
your conscience is clear
> http://www.schuerig.de/michael/ --Tanita Tikaram, Twist In My Sobriety
> _______________________________________________
> Rails mailing list
> Rails-1W37MKcQCpIf0INCOvqR/iCwEArCW2h5@public.gmane.org
> http://lists.rubyonrails.org/mailman/listinfo/rails
>

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Michael Schuerig wrote:
| On Wednesday 17 August 2005 17:16, Steve Downey wrote:
|
|>Michael Schuerig wrote:
|>
|>>Is there an elegant way to make all instances of a class derived
|>>from AchtiveRecord read-only? It must be possible to load objects
|>>from the DB, of course, but after that these objects should be
|>>immutable.

Shot in the dark, but how about removing all of the YourClass#attrmethods?

Regs,
D

|
|
|>I''m fairly new to Ruby, but would freezing the instance work?
|
|
| No, apparently it does not. As I don''t know how freezing works
exactly,
| I don''t know why.
|
| Michael
|

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David Thomson wrote:
>Does Rails not override the freeze method for it''s own purposes?
>Perhaps it aliases the original version of freeze when it does so?
>
>  
>
Yep. In base.rb:

     # Just freeze the attributes hash, such that associations are still 
accessible even on destroyed records.
      def freeze
        @attributes.freeze
      end

On 8/17/05, Michael Schuerig
<michael-q5aiKMLteq4b1SvskN2V4Q@public.gmane.org>
wrote:
> On Wednesday 17 August 2005 17:16, Steve Downey wrote:
> > Michael Schuerig wrote:
> > >Is there an elegant way to make all instances of a class derived
> > > from AchtiveRecord read-only? It must be possible to load objects
> > > from the DB, of course, but after that these objects should be
> > > immutable.
> 
> > I''m fairly new to Ruby, but would freezing the instance work?
> 
> No, apparently it does not. As I don''t know how freezing works
exactly,
> I don''t know why.
> 
> Michael
Why not add a before_save that returns false?  Have attribute readers
raise exceptions so you can''t set any data:

@attributes.keys.each do |attr_name|
  define_method(attr_name) do
    raise SomeErrorOfYourChoice
  end
end

If this works and is incredibly useful, you could even factor this
into a simple acts_as_frozen class method...

-- 
rick
http://techno-weenie.net

On Wednesday 17 August 2005 19:19, Steve Downey wrote:
> David Thomson wrote:
> >Does Rails not override the freeze method for it''s own
purposes?
> >Perhaps it aliases the original version of freeze when it does so?
>
> Yep. In base.rb:
>
>      # Just freeze the attributes hash, such that associations are
> still accessible even on destroyed records.
>       def freeze
>         @attributes.freeze
>       end
Which would be fine if I had a place to call it. Apparently there''s no 
hook that''s called for each newly created object. AR::Base#instantiate 
is private.

Michael

-- 
Michael Schuerig                            This is not a false alarm
mailto:michael-q5aiKMLteq4b1SvskN2V4Q@public.gmane.org                        
This is not a test
http://www.schuerig.de/michael/                      --Rush, Red Tide

Hello Michael,

Michael Schuerig said the following on 2005-08-17 15:26:
> Which would be fine if I had a place to call it. Apparently
there''s no
> hook that''s called for each newly created object.
AR::Base#instantiate
> is private.
That''s where you are wrong :)  There does exist a way, see "The 
after_find and after_initialize exceptions"[1].

In short, define an after_initialize method, and you should be fine.

See ya !
François

[1] http://api.rubyonrails.com/classes/ActiveRecord/Callbacks.html

On Wednesday 17 August 2005 21:31,
rails-1W37MKcQCpIf0INCOvqR/iCwEArCW2h5@public.gmane.org
wrote:
> Hello Michael,
>
> Michael Schuerig said the following on 2005-08-17 15:26:
> > Which would be fine if I had a place to call it. Apparently
there''s
> > no hook that''s called for each newly created object.
> > AR::Base#instantiate is private.
>
> That''s where you are wrong :)  There does exist a way, see
"The
> after_find and after_initialize exceptions"[1].
Oh, I don''t mind as long as someone is there to warn me :-)
> In short, define an after_initialize method, and you should be fine.
Thanks, that works now.

Michael

-- 
Michael Schuerig                       Face reality and stare it down
mailto:michael-q5aiKMLteq4b1SvskN2V4Q@public.gmane.org        --Jethro Tull,
Silver River Turning
http://www.schuerig.de/michael/