Use `@Entity` to declare that a Java class represents a persistent entity type. Choose an entity for a domain object that needs its own identity, such as an item or user. A dependent value such as an address may instead be modeled as an embeddable type. section 3.3.1 section 5.1.3 section 19.2.2
A minimal entity
`@Entity public class Item { @Id private Long id; private String name; }` `@Entity` makes `Item` persistence-capable, but it is not a complete mapping without `@Id`. By default, the class maps to a table named `ITEM`, and `id` maps to its `ID` primary-key column. Because `@Id` is on a field, the provider accesses fields directly and treats the other fields as persistent state by default. section 5.2.2 section 3.3.1
Keep the class declaration distinct from the rest of its mapping:
- Use `@Id` to expose database identity; an entity needs an identifier property. section 5.2.2
- Map relationships and dependent values separately. An entity can, for example, contain an embedded address and a collection of related entities. section 19.2.2
- Keep standard annotations beside the Java declarations they describe. Hibernate reads that metadata at startup; provider-specific annotations are available when standard JPA metadata is insufficient. section 3.3.1
Listing 19.4 The User class
Path: Ch19/querydsl/src/main/java/com/manning/javapersistence/querydsl /model/User.java @Entity @NoArgsConstructor public class User { @Id #1 @GeneratedValue(generator = Constants.ID_GENERATOR) #1 @Getter private Long id; @Embedded #2 @Getter @Setter private Address address; #2 @OneToMany(mappedBy = "user", cascade = CascadeType.ALL) #3 private Set<Bid> bids = new HashSet<>(); #3 // . . . } #1 The ID field is an identifier generated by the <kbd>Constants.ID_GENERATOR</kbd> generator. For a review of generators, revisit chapter 5. #2 The address does not have its own identity; it is embeddable. #3 There is a one-to-many relationship between <kbd>User</kbd> and <kbd>Bid</kbd>, which is mapped by the <kbd>user</kbd> field on the <kbd>Bid</kbd> side. <kbd>CascadeType.ALL</kbd> indicates that all operations will be propagated from the parent <kbd>User</kbd> to the child <kbd>Bid</kbd>.
The User mapping shows `@Entity` alongside a generated identifier, an embedded value, and a one-to-many relationship. section 19.2.2
The same JPA mappings can be used through Hibernate or Spring Data JPA. Do not confuse the annotation with persistence operations: application code still needs a configured persistence approach to store and load instances. With Hibernate OGM, entities using only JPA annotations can also be persisted to MongoDB or Neo4j by changing dependencies and persistence-unit configuration. section 5.0 section 2.8 section 18.4
