Question

I need help in retrieving data from a Spring Boot application using JPA to handle Many-to-Many relationships. I’m having trouble getting the child entities of a parent entity.

I have a simple many to many relationship established: Owner <-> Book <-> Publisher

When I do a GET on the controller to fetch data about a Book, I only get the Book’s attributes. No information about the associated Owner or Publisher is returned. What am I missing?

Executing cURL -XGET http://localhost:8080/books only returns the attributes from the book and not the dependent objects:

[
   {
      "id":1,
      "name":"Book 1",
      "isbn":"978-0743246261"
   },
   {
      "id":2,
      "name":"Book 2",
      "isbn":"978-0743246262"
   },
   {
      "id":3,
      "name":"Book 3",
      "isbn":"978-0743246263"
   },
   {
      "id":4,
      "name":"Book 4",
      "isbn":"978-0743246264"
   }
]

I’ve setup the project in GitHub and it is ready to run and test right away: https://github.com/tekpartner/learn-spring-boot-many-2-many

Answer 1

默认情况下，使用策略 lazy 进行多对多关系提取，意味着在您不调用方法从主Object检索多对多关系对象之前，hibernate将不会加载它们来自数据库。

请设置获取策略EAGER，以便hibernate在加载主对象时加载many2many关系

fetch=FetchType.EAGER

示例代码：

@ManyToMany(mappedBy = "userGroups", fetch = FetchType.EAGER)
private Set<User> users = new HashSet<User>();

Answer 2

@ bhushan-uniyal上面提到的解决方案是交换引用JsonManagedReference＆amp; JsonBackReference，它的工作原理。推送代码并且工作正常。

SpringBoot JPA Many-to-Many relationship - Rest Webservice unable to return child objects

2 个答案: