Where do you record validation rules for form data in a web application?

Question

Say you have a web form with some fields that you want to validate to be only some subset of alphanumeric, a minimum or maximum length etc.

You can validate in the c

Answer 1

To answer the actual question:

First of all it isn't allways the case that the databse restriction matches client side restrictions. So it would probably be a bad idea to limit yourself to only validate based on database constraints.

But then again, you do want the databse constraints to be reflected in your datamodel. So a first approximation would probably be to defins some small set of perdicates that is mappeble to both check constraints, system language and javascript.

Either that or you just take care to kepp the three representations in the same place so you remember to keep them in sync when changeing something.

But suppose you do want another set of restrictions used in a particular context where the domain model isn't restrictive enough, or maybe it's data that isn't in the model at all. It would probably be a good idea if you could use the same framwork used to defin model restriction to define other kinds of restrictions.

Maybe the way to go is to define a small managable DSL for perdicates describing the restriction. Then you produce "compilers" that parses this DSL and provides the representation you need.

The "DSL" doesn't have to be that fancy, simple string and int validation isn't that much of a problem. RegEx validation could be a problem if your database doesn't support it though. You can probably design this DSL as just a set of classes or what your system language provides that can be combined into expressions with simple boolean algebra.