Where To Put Validation Logic?

This is probably a question you could put to any programmer who writes
object-oriented code.

I'm developing a Mongo object mapper for Ruby, inspired by Candy's
write-on-set approach. I'm currently still in the planning phase, trying
to work out the scope of the project and the most appropriate interface.
One question I now find myself asking, is whether to include validation
helpers, and if so, where should they reside? In the model, or in the
controller? The answer I guess also depends on the scope of the project.
Will the mapper act as a foundation for all models (like most mappers),
or will it be more like a tool that models will simply use, if they have
to - something I'm still undecided on, but which I'm hoping this thread
may help me with.

While validation serves as a means to ensure the integrity of data, it
can also serve as a means to validate user input at the application
level. If you look at validations as a means to ensure data integrity,
then naturally, such logic should reside in the model; the data layer of
the application. On the other hand, looking at validation as a means to
validate user input and interactions with the UI, you would think the
controller would be the better fit.

When you think about it, you're really dealing with two types or levels
of validation: user-level, and data-level. A good example that
demonstrates this, is assume you had an application that can accept a
home phone number, a work phone number, and a mobile/cell phone number.
No field is mandatory, but at least one phone number must be given. You
could call this multi-field validation. In theory, the data layer of
your application shouldn't give a damn about whether or not at least one
phone number is set. All the data layer should care about is whether it
has a valid phone or not. How I see it, such "multi-field" validation
can be classed as general application logic which should be the
responsibility of the view/controller, not the data layer, who's only
responsible should be persisting the data.

Sadly, I don't know the right answer here, hence why I've come to this
forum. Hopefully some of you have some thoughts or opinions to share on
this matter?

Remember, one the main reasons for bringing up this topic, is because
I'm writing a Mongo object mapper that writes to Mongo when a field is
set, hence such multi-field validation can't easily be achieved. This
made me challenge my understanding of validations and which part of the
application should be responsible for them.

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