Spring Data JPA限制嵌套集合大小为Pageable

Question

I'm using SpringBoot 2.0.4.

The idea is to retrieve a Conversation entity that contains a collection of Message objects. But I need a control over the size of messages collection eg retrieve only 20 latest or so.

@Entity
public class Conversation {
    //fields...
    @OneToMany(cascade = CascadeType.ALL)
    private List<Message> messages = new ArrayList<>();
    //methods...
}

and

@Entity
public class Message {
    //....
}

So when I'm retrieving a conversation by Id like this:

conversationRepository.findById(id)

How can I include only the x (eg 20) of latest messages?

Answer 1

In the MessageRepository interface, add:

@Query("select messages from Conversation c inner join c.messages messages where c = :conversation")
  public Page < Message > findByConversation(@Param("conversation") Conversation conversation, Pageable pageable);

To return the last 20 ( or even n ) records , you sort them by message id in descending order:

Pageable pageable = new PageRequest(0, 20, Sort.Direction.DESC, "id");
Page<Message> messages = repo.findByConversation(conversation, pageable); 
List<Message> last20MessageList = messages.getContent();

Answer 2

You can try this code for your case

Pageable pageable = new PageRequest(0, 20, Sort.Direction.DESC, "id");

Page<Message> lastPage = conversationRepository.findById(id, pageable);
// this is a list of the last 20 records
List<Message> latestMessage = lastPage .getContent();

Hope this help!