[TransWarp] Constraints on model attributes

[Roché Compaan]

* Phillip J. Eby <pje at telecommunity.com> [2003-07-26 21:46]:
> At 06:43 PM 7/26/03 +0200, Roché Compaan wrote:
> >How does one define constraints for model attributes? Last time you talked
> >about this was back in January.
> 
> If you have a mdl_normalize() method on a feature's  'referencedType', it 
> is called with any value assigned to the feature (or added, if the feature 
> is a collection).  You can also define cardinality via a feature's 
> lowerBound and upperBound attributes.
> 
> Currently, to get any validation apart from cardinality and type 
> constraints, you have to define _onLink()/_onUnlink() methods, which are 
> also called when changes are made to a feature.

I've played around more and _onLink and _onUnlink methods are the
perfect place for validation and constraint hooks. I don't think
anything needs to be done to model features or datatypes. Here's a
crude example:

class TextLine(model.Attribute):

    referencedType = model.String
    title = u''
    description = u''

    def constraint(self, value):
        return '\n' not in value and '\r' not in value

    def _validate(self, element, item):

        if not isinstance(item, unicode):
            raise TypeError

        if self.constraint is not None and not self.constraint(item):
            raise ValidationError

    def _onLink(self, element, item, posn):
        self._validate(element, item)


class Person(model.Element):

    class Name(TextLine):
        title = u'Name'
        description = u'''Person's name'''

Do you see any problem with this approach?

-- 
Roché Compaan
Upfront Systems                 http://www.upfrontsystems.co.za