Quick Recap: JPA (Java Persistence API)

JPA is a standard in Java for object-relational mapping (ORM) — it allows developers to map Java objects to database tables and perform CRUD operations without writing SQL manually.

It is mainly used with relational databases (e.g., Oracle, PostgreSQL, MySQL) and can also be used with MongoDB using Spring Data.

Core Concepts in JPA

Entity → Represents a table row
Entity Manager → Handles DB operations
Persistence Context → Cache/session for entities
Repository → Interface for CRUD queries
Transaction → Ensures atomic operations

Mapping Entities (Oracle Example)

@Entity
@Table(name = "users")
public class User {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
@Column(nullable = false)
private String name;

JPA Annotations Cheat Sheet

Annotation	Purpose
`@Entity`	Marks a class as a DB entity
`@Table`	Maps to DB table
`@Id`	Primary key
`@GeneratedValue`	Auto-increment strategy
`@Column`	Map field to column
`@OneToMany`	Relationship mapping
`@ManyToOne`	Foreign key relation
`@Transactional`	Wrap method in transaction

Spring Data JPA Repository (Oracle Example)

public interface UserRepo extends JpaRepository<User, Long> {
List<User> findByName(String name);
}

👉 No SQL needed — method names generate queries.

Transactions (Relational DBs)

@Transactional
public void saveUser(User user) {
repo.save(user);
}

👉 Guarantees ACID behavior with Oracle/Postgres/MySQL.

JPA with MongoDB (Spring Data)

MongoDB is NoSQL, so traditional JPA is not used. Instead, we use Spring Data MongoDB, which follows a similar repository pattern.

Entity for MongoDB:

@Document(collection = "users")
public class User {
@Id
private String id;
private String name;
}

Repository:

public interface UserRepo extends MongoRepository<User, String> {}

👉 Still works like JPA, but no joins, no relationships, no transactions across collections.

JPA vs MongoDB – Key Differences

Feature	JPA (Oracle)	MongoDB
Database Type	Relational (SQL)	NoSQL (Document)
Schema	Fixed	Flexible
Transactions	ACID	Limited (single doc)
Joins	Yes	No (embed instead)
Querying	JPQL / Criteria	Mongo Query DSL
Repository	JpaRepository	MongoRepository