Deexterity: Validation of multiselection Field with input from Vocabulary

Hello,

I try to validate a Dexterity multiselection field that is created like
this:

category_choice = schema.List(
    title=_(u"Choose your categories"),
    value_type=schema.Choice(source=vocabCategories),
    required=True,
)


I tried to validate if this field is not empty. I wasn't successful by using @invariant with a function that raises an error message if data.category_choice is None.


@invariant
def noCategoryChoosen(data):
    if data.category_choice is None:
       raise MissingCategory(_(u"You must choose at least one category for your project"))



class MissingCategory(Invalid):
    __doc__ = _(u"You have not chosen a category for the project")


The test seemed to fail because the field has some value / data.

I tried the same with a constraint for the schema.List field with the same result.

Thanks for any hints in advance.

Regards,
Andreas

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