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JPA

What it means to store objects in tables: the persistence context, relationships, lazy loading, and when and how many queries are sent.

Getting started

• What JPA solves — When you write the SQL yourself, what do you have to fill in by hand?

Persistence context

• Persistence context — When does JPA send SQL?
• The lifecycle of an entity — Why is the same setName sometimes not applied to the DB?
• Detached entities and merge — Why were other values erased when I only fixed the name with merge?

Entity mapping

• Entity mapping and primary keys — How do the annotations on an entity change the table and the values that are stored?
• Value types and embeddable types — How is a class that groups an address stored in a table, and why make it immutable?

Relationships

• The owning side of a relationship — You added a member to the team's list, so why is the foreign key in the database empty?
• Many-to-one, one-to-many, one-to-one, many-to-many — How do the tables and foreign keys change depending on the kind of relationship?
• Inheritance mapping — In how many tables, and how, are inherited classes stored?
• Cascade and orphan removal — What happens to the children when you save or delete only the parent?

Proxies and fetching

• Proxies and lazy fetching — Is an object received through lazy fetching a real entity?
• N+1 — Why are many queries sent when you load the order list only once?

Queries

• JPQL and fetch joins — What is the difference between join and join fetch?
• Bulk update and delete operations — After one UPDATE changes many rows, why does the entity still have the old value?
• Spring Data JPA — How does a repository work when there is no implementation class?
• Dynamic queries and Querydsl — How do you add a search condition to where only when it is given?

Web applications and performance

• Connection pool and OSIV — Why connections run short even though the queries finish quickly
• Optimistic and pessimistic locking — What if many orders arrive at once for 3 items in stock?
• Querying directly into a DTO — What is the difference between querying entities and copying them, and querying directly into a DTO?