JPA Entity Relationships
https://www.tutorialspoint.com/jpa/jpa_entity_relationships.htm

JPA Entity Primary Key
http://www.objectdb.com/java/jpa/entity/id#Composite_Primary_Key_

JPA Many-To-Many Relationship Mapping Example with Spring Boot, Maven and MySQL
https://hellokoding.com/jpa-many-to-many-relationship-mapping-example-with-spring-boot-maven-and-mysql/

How To Solve JSON infinite recursion Stackoverflow
http://keenformatics.blogspot.pt/2013/08/how-to-solve-json-infinite-recursion.html

Infinite Recursion with Jackson JSON and Hibernate JPA issue
http://stackoverflow.com/questions/3325387/infinite-recursion-with-jackson-json-and-hibernate-jpa-issue

Spring Web security
https://www.youtube.com/watch?v=XLfOlqG_tNU&list=PLGDwUiT1wr6-cvT21QHjfB_9xf7b7k7a-&index=1
http://tuhrig.de/expression-based-security-with-spring-security/