I would expect the promiseLibrary to be specified once for the Db instance, and then passed down automatically, and that happens most of the time, except in this scenario:

mongodb.MongoClient.connect("mongodb://localhost/test",

{ promiseLibrary: require('bluebird') }

).then(function(db)

{ return db.createCollection("test"); }

).then(function(col)

{ // This fails. assert(col.s.options.promiseLibrary); }

);