从孩子保存时，休眠一对多双向连接会提供新的父母

问题描述

我正在尝试通过 freemarker 将子项添加到预先存在的父项中。但是每次尝试时，都会出现以下错误：

org.postgresql.util.PSQLException: ERROR: insert or update on table "CHILD TABLE" violates foreign key constraint "KEY" Detail: Key (id)=(WRONG NUMBER) is not present in table "PARENT TABLE".

我似乎每次都按顺序给出一个新的父 id，而不是在调用 childRepository.save(child); 之前给出并出现在子对象中的父 id。

代码片段：

家长：

@Entity
@Table(name = "parent")
@EntityListeners(AuditingEntityListener.class)
public class Parent {

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private long id;

@OneToMany(mappedBy = "parent")
private Set<Child> children = new HashSet<Child>();

孩子：

@Entity
@Table(name = "child")
@EntityListeners(AuditingEntityListener.class)
public class Child{

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private long id;

@ManyToOne
@JoinColumn(name = "parent_id",nullable = false)
private Parent parent;

Freemarker 控制器：

 @Autowired
    private ChildRepository childRepository;
    
    @RequestMapping("/addchild/{id}")
    public String addChild(Model model,@PathVariable(value = "id") Long parentId) {
        String method = "addChild";
        Utils.log(method,"started");
        
        parent parent= parentRepository.findById(parentId)
                .orElseThrow(() -> new IllegalArgumentException("parent " + parentId + " not found"));
        Utils.log(method,"loaded " + parent);
        child child = new child();
        child.setparent(parent);
        
        model.addAttribute("child",child);
        return "child";
    }
    
    @PostMapping("/savechild")
    public String savechild(Model model,child child) {
        String method = "savechild";
        Utils.log(method,"started");
        
        child.getparent().addchild(child);
        
        child = childRepository.save(child);
        
        Utils.log(method,"saved " + child);
        
        return "redirect:/parent/" + child.getparent().getId();
    }

解决方法

我没有使用过 Freemarker，但 PostMapping 代码在我看来就像一个 Spring MVC 控制器，其中包含基于误解的错误代码。看， PostMapping 从例如获取一些输入一个 HTML 表单，并构造一个子实例，根据表单输入设置所有字段。

但由于 HTTP 是一种无状态协议，控制器方法绝对知道的只是您输入的内容，并且您从之前的 GET 调用中获得的父信息完全丢失了。因此，要在 POST 上添加子项，您还必须首先获取父项。

我本来希望看到类似的东西

@Autowired private ParentRepository parentRepository;
@Autowired private ChildrenRepository childrenRepository;

@RequestMapping("/parents/{parentId}/children/{childId}")
public String getChild(Model model,@PathVariable(value = "parentId") Long parentId,@PathVariable(value = "childId") Long childId) {

    // Fetch a specific child for a specific parent fully determined by URL
    Child child = childrenRepository.findByParentIdAndId(parentId,childId).orElseThrow(() -> new IllegalArgumentException("parent " + parentId + "child " + childId + " not found"));
   
    model.addAttribute("child",child);
    return "child";
}

// The parent to add to is referenced by its url,the child constructed by form data
@PostMapping("/parents/{id}")
public String saveChild(Model model,@PathVariable(value = "id") Long parentId,child child) {
    // Fetch the parent you want to add a child to
    parent parent = parentRepository.findById(parentId)
            .orElseThrow(() -> new IllegalArgumentException("parent " + parentId + " not found"));
    // Add the child constructed from form data and send to parent specific url
    parent.addchild(child);
    // Save parent with updated list of children
    parentRepository.save(parent)
    //
    return "redirect:/parents/" + parent.getId();
}

,

错误是一个错误的约束，可能是早期版本代码的工件