-> parent to child
<-> embedding, child referencing  

tours -> reviews 
users -> review
location <-> tours
tours <-> users
tours -> bookings
users -> bookings

https://documenter.getpostman.com/view/2054414/Szt8eq8B

sendgrid only works for production env, local uses mailtrap
https://whispering-mountain-15129.herokuapp.com/

heroku login
git commit -m "" && git push heroku master
heroku config:set STRIPE_WEBHOOK_SECRET=yourkey
heroku logs

await query.explain() - look for executionstats and amount of document examined

tourSchema.index({ price: 1, ratingsAverage: -1 });
tourSchema.index({ slug: 1 });
tourSchema.index({ startLocation: '2dsphere' });

indexing consume rource on create and update. Have to compare frequency of read write ratio before creating index