having some validation logic in abstract class

Hi all, 

The question is how to add some validation statement in an upper/abstract class against all instances of subclasses.

here is the model

class UpperAbstract(models.Model):

  def verifyUnique(value):

    if value:

      len(UpperAbstract.objects.filter(myuniquefield=value)) > 0:

         raise ValidationError(...)

  myuniquefield = CharField(max_length=15, blank=True,validators=[verifyUnique]))

  class Meta:

abstract = True

class Foo(UpperAbstract):

      myFoofield = CharField(max_length=15)

class Bar(UpperAbstract):

      myBarfield = CharField(max_length=15)

Validation and store of any instance of Foo or Bar is failing as UpperAbstract do not have objects attribute.

I can imagine why  but my question is then how to achieve this type of validation ? 

PS : my first goal was to be able to define field as 'unique if not None'.

thank in advance 

Regards

manu

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