promise example for mongoose

mongoose_promise.js

var relationPromise = ArticleRelation.findOne({parentArticleGuid: oldArticle.guid}).exec();
 
relationPromise
.then(function(relation){
	if (relation == null){
		 relation = new ArticleRelation({
		 	parentArticleGuid: oldArticle.guid,
		 	organization: task.organization._id
		 });
	}

	return relation;
}
.then(function(relation){
	relation.save(function (error, articleRelation) {
		if (error) {
			throw error;
		}
	})
}, function(err){
	throw err;
});