Mongoose schemas and collections
I solved all my doubts taking a look into this tutorial, as you can see in here they implement two collections within one database (My inicial concern). Once you are connected to the database and you perform a post to the collection you want, in this case thread or post, it will create the collection into mongodb.
The issue there is that your connection string is mongodb://localhost/song
Song becomes your DB, and within Song you should be able to see two collections - SongS - ArtistS
Posible solutions: Flush the database, drop all. Start clean and check what your application is doing. Use another name for the DB