Flask Framework Cookbook - Second Edition

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We just saw how to write custom fields. Similarly, we can write custom validations, too. Let's assume that we do not want to allow duplicate categories. We can implement this in our models easily, but let's do this using a custom validator on our form:

from wtforms.validators import ValidationError def check_duplicate_category(case_sensitive=True): def _check_duplicate(form, field): if case_sensitive: res = Category.query.filter( Category.name.like('%' + field.data + '%') ).first() else: res = Category.query.filter( Category.name.ilike('%' + field.data + '%') ).first() if res: raise ValidationError( 'Category named %s already exists' % field.data ) return _check_duplicate class CategoryForm(NameForm): name = StringField('Name', ...