set mongo.MongoClient.connect() once and have all your app.get() inside of it doing whatever database operations (not sure where to put the db.close() given async nature); or
have individual mongo.MongoClient.connect() within each app.get(). Perform whatever operation with the database and db.close() it each time within the app.get()
As far as I know this is considered best practice when using mongodb (not Mongoose):
// Make db connection
db.connect(process.env.DB_URL, function(err){
// Log an error if one occurs
if(err){
console.log('Unable to connect to MongoDB');
process.exit(1);
}
// Start the app if the db connection was succesfull
else{
app.listen(port, function(){
console.log('App listening on port', port);
});
}
});