Spring Data REST is a framework for building RESTful APIs on top of Spring Data repositories. It allows you to expose your data entities as a RESTful API by simply adding a few annotations to your entity classes and repositories.

One of the key features of Spring Data REST is its support for working with relationships between entities. You can define relationships between your entities using JPA annotations, and Spring Data REST will automatically expose these relationships as links in the RESTful API.

For example, consider the following two entities:

@Entity
public class Customer {
@Id
@GeneratedValue
private Long id;
private String name;
@OneToMany(mappedBy = "customer")
private List<Order> orders;
// ...
}

@Entity
public class Order {
@Id
@GeneratedValue
private Long id;
@ManyToOne
private Customer customer;
// ...
}

In this example, the Customer entity has a one-to-many relationship with the Order entity, as defined by the @OneToMany and @ManyToOne annotations. When you expose these entities as a RESTful API using Spring Data REST, the API will automatically include links to related resources. For example, you might see a response like this:

{
"id": 123,
"name": "John Smith",
"orders": [
{ "id": 1, "customer": { "id": 123, "href": "http://example.com/customers/123" } },
{ "id": 2, "customer": { "id": 123, "href": "http://example.com/customers/123" } }
]
}

This response includes a list of orders that are related to the customer, and each order includes a link to the related customer resource. You can use these links to navigate the relationships between your entities, making it easy to build APIs that expose complex data structures.

There are many other features and options available when working with relationships in Spring Data REST, such as configuring the names of the links, exposing nested relationships, and defining custom controllers to handle specific cases. You can learn more about these features in the Spring Data REST documentation.