OO design patterns to use for validation

Question

I am in the process of writing some validation code based on these assumptions:

• Validation code must be in an external class
  • i.e. no data class contains it's own validation
• The same object can be validated in different ways
  • e.g. validate syntax only; validate against DB look-ups; validate against duplicates; etc
• Validation output can be different depending on what needs it
  • e.g. output a single error message; output a list of all validation errors; similar but in JSON format and including error codes; etc

What combination of OO design patterns are best to solve this? A factory might be a good way to get a specific validator, but are their better approaches?

Answer 1

One size does not fit all! Make it simple!

Provide validators with common methods/interface to output data, categorize warnings, filter/process warnings raised more than once. Do not create any sophisticated way of validation itself, at least not before writing a few real life validators.

Move out of the way and let the validators do what they are supposed to do:

for validator in all_validators:
    validator.validate(model)

Answer 2

I think I am doing the same thing right now.
The pattern that applies here is the Filter pattern and the Filter Chain.

Each filter validates against one "way" (as you call them).
First for syntax, second for Db lookups etc (from your second bullet).