Recently I accidentally had a typo in my Mongoid Rails model where I specified a collection name to store the documents in.
After playing around with my app I discovered my mistake - and having existing associations by now - I wanted to fix this. So I removed the typo from the code and at the end I had to rename the collection within MongoDB.
Let's say the wrong collection name was wordifycsmexternal_links and the correct name wordifycmsexternal_links. I fired up my Mongo shell, select the database and renamed the collection: