'Make sure that there\'s only one case per person and disease'

@classmethod
def case_create_onvalidation(cls, form):

    formvars = form.vars
    try:
        case_id = formvars.id
        person_id = formvars.person_id
    except AttributeError as e:
        return
    if ('disease_id' not in formvars):
        disease_id = current.s3db.disease_case.disease_id.default
    else:
        disease_id = formvars.disease_id
    record = cls.get_case(person_id, disease_id)
    if (record and (record.id != case_id)):
        error = current.T('This    case    is    already    registered')
        link = A(record.case_number, _href=URL(f='case', args=[record.id]))
        form.errors.person_id = XML(('%s:    %s' % (error, link)))
    return
