Rails - Mar 2006 - One-way change

I''m developing an app, and I want to make a single boolean field OK to 
change from 0 to 1, but not allow 1 to 0.

Seems like the model''s validate method would be the right place for 
this, but I''m not sure how to implement it.

Any suggestions (actual or pointers to) are greatly appreciated.

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On Mon, Mar 06, 2006 at 04:01:46AM +0100, Chris Schumann wrote:
} I''m developing an app, and I want to make a single boolean field OK
to
} change from 0 to 1, but not allow 1 to 0.
} 
} Seems like the model''s validate method would be the right place for 
} this, but I''m not sure how to implement it.
} 
} Any suggestions (actual or pointers to) are greatly appreciated.

This is the sort of thing I''d do in a database trigger. When you
attempt to
save, the trigger will get activated and throw a hissy if necessary, which
will cause ActiveRecord to tell you that the save failed.

--Greg

> This is the sort of thing I''d do in a database trigger. When you
attempt
> to save, the trigger will get activated and throw a hissy if necessary, 
> which will cause ActiveRecord to tell you that the save failed.
> 
> --Greg
That sounds like it would work... and like something I don''t want to
do.
There has to be a way to do it in Rails. I just thought it might be an 
easy thing.

I suppose I could put it in the view that if the flag is set, don''t put
up a field for a new entry. That doesn''t seem so awful now that I think
about it, except that the rule should go in the model.

def validate_on_update
  oldobj = find(obj.id);
  if (oldobj.field == 1) and (field == 0)
    errors.add("field", "cannot be cleared once set")
  end
end

I know I can get the new values, but is the old object around? I could 
go get the record as it''s already stored, but it seems it might already
exist somewhere.

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On Mon, Mar 06, 2006 at 04:03:06PM +0100, Chris Schumann wrote:
} > This is the sort of thing I''d do in a database trigger. When you
attempt
} > to save, the trigger will get activated and throw a hissy if necessary, 
} > which will cause ActiveRecord to tell you that the save failed.
} > 
} > --Greg
} 
} That sounds like it would work... and like something I don''t want to
do.
} There has to be a way to do it in Rails. I just thought it might be an 
} easy thing.
} 
} I suppose I could put it in the view that if the flag is set, don''t
put
} up a field for a new entry. That doesn''t seem so awful now that I
think
} about it, except that the rule should go in the model.
} 
} def validate_on_update
}   oldobj = find(obj.id);
}   if (oldobj.field == 1) and (field == 0)
}     errors.add("field", "cannot be cleared once set")
}   end
} end
} 
} I know I can get the new values, but is the old object around? I could 
} go get the record as it''s already stored, but it seems it might
already
} exist somewhere.

Well, your other option is to override the field= method in the model. For
example, suppose your field is named foo:

def foo=(value)
  fail "Goodbye, cruel world!" if foo == 1 && value == 0
  super(value)
end

Of course, this assumes that the only way the value of foo is modified is
through the mutator method, and I believe that is the case.

--Greg

> Well, your other option is to override the field= method in the model. 
> For example, suppose your field is named foo:
> 
> def foo=(value)
>   fail "Goodbye, cruel world!" if foo == 1 && value == 0
>   super(value)
> end
> 
> Of course, this assumes that the only way the value of foo is modified 
> is through the mutator method, and I believe that is the case.
Brilliant. This should do the job exactly. Thanks!

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