Lecture 4A

Question 1

What is a an embedded id?

Answer 1

Entire object in a field is used as primary key: @EmbeddedId DependentId id; A variant of composite key. Also original class must be @Embeddable

Question 2

How to auto generate id

Answer 2

@Id @GeneratedValue

Int id;

Question 3

How update existing entity?

Answer 3

em.merge(customer);
If there’s already a customer with same primary key merge updates it, otherwise it persists it. Must be embedded in transaction.

Question 4

Explain process of em.find(customer.getClass(), id);

Answer 4

If class and id refers to an entity currently in PC this entity is returned from cache. If the entity is not in PC but is in database a new managed object is created in PC and returned. Otherwise null is returned.

Question 5

How to write compound primary key in JPA?

Answer 5

@Id int id;

@Id String name;

Question 6

JPQL ?

Answer 6

Java Persistence Query Language. Allows for more sophisticated queries. Based on older Hibernate Query Language. Operates on entities(objects) rather than tables.

Question 7

Do basic JOQL search

Answer 7

SELECT c FROM Country c

Question 8

Do JPQL queries need a transaction?

Answer 8

No. Example: Query myQuery = em.createQuery(“SELECT c FROM Country c”);
List results = query.getResultList();

Question 9

How to make a query typed?

Answer 9

TypedQuery q = em.cresteQuery(“SELECT c FROM Country c”);

List results = q.getResultList();