# Why validation occurs on attribute read instead of write?

**URL:** <https://www.ruby-forum.com/t/why-validation-occurs-on-attribute-read-instead-of-write/72371>\
**Category:** Rails\
**Created:** [October 16, 2006, 8:50pm UTC](https://www.ruby-forum.com/t/why-validation-occurs-on-attribute-read-instead-of-write/72371 "2006-10-16T20:50:55Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![weyus](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/7ab992/32.png) [@weyus](https://www.ruby-forum.com/u/weyus)\
**Post date:** [October 16, 2006, 8:50pm UTC](https://www.ruby-forum.com/t/why-validation-occurs-on-attribute-read-instead-of-write/72371/1 "2006-10-16T20:50:55Z")

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I have what I think is a simple question.

Validation appears to occur upon the read of an ActiveRecord attribute,  
not on the write of that attribute.

Assuming a case where you may have some transformation that needs to  
happen to the data on the way in, this can create situations where if  
your inbound transformation inadventertently changes your data such that  
it “becomes” valid, then unless you are careful to validate that data on  
the write, a form could appear to allow invalid data.

Granted, I do believe this can only occur if the inbound transformation  
itself allows successful data tranformation in cases that should have  
been rejected by the validation. Basically, the setters need to be  
aware of the validations as well.

A concrete example is for fax numbers that are stored internally as only  
digits. The form may accept a few formatting characters however. The  
validation should reject any invalid characters. If the setter for the  
fax number strips out all of the character data before the call to  
write\_attribute(), then the validation, which is looking for _invalid_  
characters doesn’t get tripped on the read back out and the object  
updates. It then appears that the user can successfully input fax  
numbers with invalid data.

So my question is, why aren’t the validation rules applied whenever an  
AR setter is invoked instead of when the getter is invoked? It seems  
that you would have to assume that all of the setters were of the form:

def set\_method=(new\_set\_value)  
…  
end

meaning only one input parameter, and then this input parameter would be  
what is validated.

What am I missing here?

Thanks,  
Wes

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<div class="post-metadata">

**Author:** ![weyus](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/7ab992/32.png) [@weyus](https://www.ruby-forum.com/u/weyus)\
**Post date:** [October 16, 2006, 8:58pm UTC](https://www.ruby-forum.com/t/why-validation-occurs-on-attribute-read-instead-of-write/72371/2 "2006-10-16T20:58:30Z")

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> So my question is, why aren’t the validation rules applied whenever an  
> AR setter is invoked instead of when the getter is invoked? It seems  
> that you would have to assume that all of the setters were of the form:
> 
> def set\_method=(new\_set\_value)  
> …  
> end
> 
> meaning only one input parameter, and then this input parameter would be  
> what is validated.

Just to clarify:

def attr\_name=(new\_attr\_value)  
…  
end

_is_ the default attribute setter for a given AR attribute.

It seems like you could apply the validation to the input to this method  
instead of to the output of the related getter method.

Thoughts?  
Wes

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<div class="post-metadata">

**Author:** ![weyus](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/7ab992/32.png) [@weyus](https://www.ruby-forum.com/u/weyus)\
**Post date:** [October 17, 2006, 12:58am UTC](https://www.ruby-forum.com/t/why-validation-occurs-on-attribute-read-instead-of-write/72371/3 "2006-10-17T00:58:53Z")

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Oh, and of course if you need to check whether an object is valid, you  
can call the valid? method.

Fred

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<div class="post-metadata">

**Author:** ![weyus](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/7ab992/32.png) [@weyus](https://www.ruby-forum.com/u/weyus)\
**Post date:** [October 17, 2006, 12:17am UTC](https://www.ruby-forum.com/t/why-validation-occurs-on-attribute-read-instead-of-write/72371/4 "2006-10-17T00:17:17Z")

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Validation occurs when the record is saved. Validating as each attribute  
is updated could be quite limiting.

Sure you could still express things like email\_address needs to match a  
certain format and blah needs to be a number but you couldn’t have more  
complex rules where various attributes interact, for example a business  
rule that states that if attribute x is set then y must be a positive  
integer, and if x is not set then y should be nil.

I guess what I’m trying to say is that being valid or not is a property  
of the object as a whole, not of any individual attribute and so it will  
not always be the case that changing a series of attributes produces an  
object which is valid at each intermediate stage

Fred