Return all unique namespaces

		:returns: a list of all predicates 
		:rtype: list of ckan.model.semantictag.Predicate objects

def list_unique(cls):
		'''Return all unique namespaces

		:returns: a list of all predicates 
		:rtype: list of ckan.model.semantictag.Predicate objects

		'''
		query = meta.Session.query(Predicate).distinct(Predicate.namespace)
		return query.all()