What effect does the @embedded annotation have?

Question

How does inline annotation affect the database?

How should SQL queries change?

What is a typical utility for using annotations?

Answer 1

How does inline annotation affect the database?

This does not affect him at all. At the ORM provider level, all fields from the inline object are merged with the parent object and are processed as if they were declared there all the time. In other words, it works as if you literally copied all fields, receivers, and setters into an object containing an embedded object.

How should SQL queries change?

They will not. You do not need to change anything. See above.

What is a typical use case for annotation annotations?

Sometimes you have a huge table with several columns (especially with legacy databases). However, some columns are logically related to each other (for example, street, city and phone number in the CUSTOMER table). If you do not want to create an object with all fields, you create a built-in Address object. This way you logically group the address columns into an object instead of having the same number of POJOs with a flat list of fields.

Using embedded objects is considered good practice, especially when a strong 1-1 relationship is found.

Answer 2

extending answer @ Tomasz Nurkiewicz Embedded objects are useful for mapping a table to a composite primary key using the @EmbenddedId annotation

Answer 3

What is a typical utility for using annotations?

Usually this represents the composite primary key as an embedded class:

 @Entity public class Project { @EmbeddedId ProjectId id; : } @Embeddable Class ProjectId { int departmentId; long projectId; } 

Primary key fields are defined in a nested class. The object contains one primary key field, which is annotated using @EmbeddedId and contains an instance of this embedded class. When using this form, a separate identifier class is not defined, since the embedded class itself can represent the full values ​​of the primary key.

How does inline annotation affect the database?

This is not true. Use this annotation to represent a composite primary key.

How should SQL queries change?

They will not.

Answer 4

As Tomas said that one goal - another - you can "photograph" the state of another related object inside your table. Fe

 @Embeddable public class Company { String name; String streetName; String city; } @Entity public class Invoice { @Embedded @AttributeOverrides({ @AttributeOverride(name="name", column=@Column (name="name")), @AttributeOverride(name="streetName", column=@Column (name="streetName")), @AttributeOverride(name="city", column=@Column (name="city")), }) Company seller; @Embedded @AttributeOverrides({ @AttributeOverride(name="name", column=@Column (name="name")), @AttributeOverride(name="streetName", column=@Column (name="streetName")), @AttributeOverride(name="city", column=@Column (name="city")), }) Company customer; } 

in this example - without the built-in and @AttributeOverrides, any change in the future client of the company will change the data in Invoice - which is a mistake - an account was generated for the company with old data.

This is well explained here: :) Java - JPA @Basic and @Embedded annotations

Answer 5

This does not always have to be a class identifier. In Domain Driven Design, you can create a component from some properties of an object, for example. in this example http://yaarunmulle.com/hibernate/hibernate-example/hibernate-mapping-component-using-annotations-1.html the student has an address component.

The Address property for the student is annotated with @Embedded to point to the component of the Address class.