I think I'm pretty much in agreement with this advice, primarily because of your last line: Some means of annotating constraints is the solution, but what form it should take is a hard question.

That's the million dollar question that most cannot solve for any reasonbly complex system. For constraints which are clearly global (your password can never be longer than 32 characters), then it's OK to trap and report them in client code. The problem with allowing the database errors to bubble up is that you theoretically want to parse the error message to make it more understandable to a user.

For my work, I want to follow that Pareto rule: 80% of your results stem from 20% of your actions. With this, if I can properly identify the most common errors users are likely to commit, I focus on eliminating the possibility then can commit these (give them a country drop down intead of a text field where they can type the country name) or give them a useful error message for the common cases rather than ever hit the database. Those situations, when properly identified and combined with an intuitive interface, should ameliorate the bulk of application problems. For situations you've not anticipate/covered, you'll have to figure out a reasonable solution and what that solution is generally needs to be customized for an application as "one size fits all" approaches generally mean "one size fits none".

Fortunately, with a well-designed database, many of the constraints are easy to identify. One-to-many relationships imply a select list if you're using HTML. Length constraints in fields imply that you can easily push those constraints to forms (try entering long passwords on Perlmonks, for example, even though you can theoretically enter a longer one). Data level constraints (age must be greater than 20 and less than 65) are more difficult to handle in a generic matter. Whether or not it's worth the programmers time to handle all of those depends upon your business needs.

Don't take this as criticism, though. I completely agree with what you're saying. I'm just filling in some detail :)

Cheers,
Ovid

New address of my CGI Course.

In reply to Re^3: Mapping database errors to user errors by Ovid
in thread Mapping database errors to user errors by jplindstrom

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