Return the predicate with the given id, or None.

		:param predicate_id: the id of the predicate to return
		:type predicate_id: string

		:returns: the predicate with the given id, or None if there is no predicate with
			that id
		:rtype: ckan.model.semantictag.Predicate

def by_id(cls, predicate_id, autoflush=True):
		'''Return the predicate with the given id, or None.

		:param predicate_id: the id of the predicate to return
		:type predicate_id: string

		:returns: the predicate with the given id, or None if there is no predicate with
			that id
		:rtype: ckan.model.semantictag.Predicate

		'''
		query = meta.Session.query(Predicate).filter(Predicate.id==predicate_id)
		query = query.autoflush(autoflush)
		return query.first()