What does this code offer for you to learn?

Security

• SignUp / SignIn mechanisms.
• Sending activation emails upon signUp.

Endpoint Validation

• Endpoint validation using Java bean annotation.

Database Connectivity in Dev/Prod Environments

• Connect to a H2 database for development
• Connect to a PostgreSQL database on a Heroku server for production.

Modeling Database Entities And Their Relationships

Adding / Deleting / Updating Entities In The System

Unit Tests