Rails - May 2005 - "validation" on destroy

I''m building a little application with Rails, and I''m becoming
a huge
fan of the validations in ActiveRecord.  Recently, though I ran into a
problem which to me feels like a job for Validations, but I can''t see
a way to use them.

My model consits of a Trade object, which represents either buying or
selling some security.  I need to require that at no point has a user
sold more than he''s bought (no short selling).  This is fantastically
easy to accomplish when entering a new trade, and when modifying
existing trades, I just add a simple validate in my Trade object. 
Unfortunately, it''s possible to get around the validation by deleting
a prior trade, and I can''t figure out the best way to deal with this.

My first thought was to override before_destroy, but it doesn''t look
like there''s a way to write code that will abort the destroy if
it''s
not legal, in the same way that I can for create and update.   I could
handle this in the controllers, of course, but this is a very
model-related problem, and letting the controllers worry about it
feels like a hack to me.

Do I need to override the destroy method itself, or am I missing something?

Thanks in advance,

Cameron

Cameron,

     Here''s one suggestion, though there may be an easier way. You  
could add a mixin, or extend the validations module via a  
module_eval, and alias the destroy method to your own that calls valid?.

     I would suggest then also adding a custom validate_on_destroy  
handler - see version 1.0.7 of the salted login generator for how I  
do this. You may want to consider proposing an official  
validate_on_destroy handler, because this sounds like it may be  
generally useful enough that other people would be interested.

Joe

On May 8, 2005, at 2:53 PM, Cameron Price wrote:
> I''m building a little application with Rails, and I''m
becoming a huge
> fan of the validations in ActiveRecord.  Recently, though I ran into a
> problem which to me feels like a job for Validations, but I can''t
see
> a way to use them.
>
> My model consits of a Trade object, which represents either buying or
> selling some security.  I need to require that at no point has a user
> sold more than he''s bought (no short selling).  This is
fantastically
> easy to accomplish when entering a new trade, and when modifying
> existing trades, I just add a simple validate in my Trade object.
> Unfortunately, it''s possible to get around the validation by
deleting
> a prior trade, and I can''t figure out the best way to deal with
this.
>
> My first thought was to override before_destroy, but it doesn''t
look
> like there''s a way to write code that will abort the destroy if
it''s
> not legal, in the same way that I can for create and update.   I could
> handle this in the controllers, of course, but this is a very
> model-related problem, and letting the controllers worry about it
> feels like a hack to me.
>
> Do I need to override the destroy method itself, or am I missing  
> something?
>
> Thanks in advance,
>
> Cameron
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