adding validates() decorator after class creation

[lars van gemerden]

Hi,

I am trying to use validates() to dynamically add validation to my sqla classes, but i cannot figure out how to do this. I have read python decorator docs and docs n validates, but something escapes me (or it is just not possible).

A code snippet (this does not work):

    def create_validators(self):      

        for cls in self.itervalues():

            names = tuple([fld.key() for fld in cls._type_.fields()])

            def validator(obj, name, value):

                if not cls._type_[name].validate(value):

                    raise ValueError("incorrect value '%s' for '%s' in '%s'" % (str(value), name, cls.__name__))

                return value

            setattr(cls, 'validator', validates(*names)(validator)) #does NOT work

in which:

'self' is a dictionary of sqla classes, 

'cls' is an sqla class (derived from declarative_base(...)), 

'cls._type_' contains the information for (further) generation of 'cls' (e.g. cls._type_[name].validate(value) calls a validate method for value for a field (say String(50)) with name 'name')

The method above runs without problem, but the validator function is never called when setting field values. I have tried all alternatives i can think of, but maybe i am missing something basic.

Can anyone help?

Cheers, Lars

[Michael Bayer]

@validates is a decorator that's typically associated with a class before it's mapped.   It is essentially a mapper-time directive which is converted into an attribute event.

If you're trying to associate these events with class attributes en-masse, and particularly that may already be mapped, you'd want to use the event API directly and establish attribute events:

from sqlalchemy import event

def create_validator(cls):

def validate(target, value, oldvalue, initiator):

   if not cls._type_[name].validate(value):

       raise ValueError("incorrect value '%s' for '%s' in '%s'" % (str(value), name, cls.__name__))

   # note no return value is needed by default

return validate

def create_validators(self):      

   for cls in self.itervalues():

       for fld in cls._type_.fields():

           attribute = getattr(cls, fld.key())

           event.listen(attribute, "set", create_validator(cls))

note also that in your original code, the "def validator()" will not refer to the correct value of "cls" in most cases as the "cls" name changes due to the loop.   Hence the need for the create_validator() call.

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[lars van gemerden]

Perfect (almost ;-), thanks a lot!

for future reference, the attribute name (key) was needed in create_validator(cls) to access the correct validate method:

    @staticmethod            

    def _create_validator(key, cls): 

        def validate(target, value, oldvalue, initiator):

            if not cls._type_[key].validate(value):

                raise ValueError("incorrect value '%s' for '%s' in '%s'" % (str(value), key, cls.__name__))

        return validate

    def create_validators(self):      

        for cls in self.itervalues():

            for fld in cls._type_.fields():

                key = fld.key()

                attribute = getattr(cls, key)

                event.listen(attribute, "set", self._create_validator(key, cls))

                debug_print('created validator for %s in %s' % (key, cls.__name__))