One-way change

Rails
1

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.

2

On Mon, Mar 06, 2006 at 04:01:46AM +0100, Chris S. 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

3

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.

4

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!

5

On Mon, Mar 06, 2006 at 04:03:06PM +0100, Chris S. 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