when changing author, remove article from previous author’s article array

controllers/articles.js

@@ -56,6 +56,10 @@ router.get('/:id/edit', function(req, res){
 });
 
 router.put('/:id', function(req, res){
+	Author.findOne({'articles._id':req.params.id}, function(err, previousAuthor){
+		previousAuthor.articles.id(req.params.id).remove();
+		previousAuthor.save();
+	});
 	Author.findById(req.body.authorId, function(err, foundAuthor){
 		Article.findByIdAndUpdate(req.params.id, req.body, {new:true}, function(err, updatedArticle){
 			foundAuthor.articles.push(updatedArticle);