Spring JPA - 多对一，没有完整的异物 [英] Spring JPA - Many to one without full foreign object

问题描述

我在 ManyToOne >这个回购。实体之间的关系是一个 Cart （购物车）可以有许多 Item s（在本教程）。

<

  @Entity 
 @Table（name =CART）
公共类Cart 
 {
 @Id 
 @GeneratedValue（strategy = GenerationType.IDENTITY）
 @Column（name =cart_id）
私人长ID; 
 
 
 @OneToMany（mappedBy =cart）
 private Set< Item>项目; 
 
 / *其余班级* / 
  

strong>

  @Entity 
 @Table（name =ITEMS）
 public class Item 
 
 @Id 
 @GeneratedValue（strategy = GenerationType.IDENTITY）
 @Column（name =id）
私人长ID; 
 
 @Column（name =item_name）
 private String itemName; 
 
 @ManyToOne 
 @JoinColumn（name =cart_id，nullable = false）
私人购物车; //需要一个整个Cart实例:( 
 
 / *其余的类* / 
  

问题

如何将项目映射到购物车 code>类，而不会使 Item 类的整个 Cart 对象部分。

我想看看 Item 类看起来像这样

  @Entity 
 @Table（name =ITEMS）
 public class Item 
 {
 @Id 
 @GeneratedValue（strategy = GenerationType .IDENTITY）
 @Column（name =id）
私有长ID; 
 
 @Column（name =item_name）
 private String itemName; 
 
 
 @ManyToOne 
 @JoinColumn（name =cart_id，nullable = false）
私人长卡车ID; //只是购物车的ID，而不是整个购物车对象
  

这给出错误

 造成者：org.hibernate.AnnotationException：com.exa上的@OneToOne或@ManyToOne mple.demo.Item.cartId引用一个未知的实体：long 
在org.hibernate.cfg.ToOneFkSecondPass.doSecondPass（ToOneFkSecondPass.java:97）〜[hibernate-core-5.2.12.Final.jar：5.2。 12.Final] 
 at org.hibernate.boot.internal.InFlightMetadataCollectorImpl.processEndOfQueue（InFlightMetadataCollectorImpl.java:1752）〜[hibernate-core-5.2.12.Final.jar：5.2.12.Final] 
在org.hibernate.boot.internal.InFlightMetadataCollectorImpl.processFkSecondPassesInOrder（InFlightMetadataCollectorImpl.java:1696）〜[hibernate-core-5.2.12.Final.jar：5.2.12.Final] 
 at org.hibernate.boot .internal.InFlightMetadataCollectorImpl.processSecondPasses（InFlightMetadataCollectorImpl.java:1583）〜[hibernate-core-5.2.12.Final.jar：5.2.12.Final] 
 at org.hibernate.boot.model.process.spi。 MetadataBuildingProcess.complete（MetadataBuildingProcess.java:278）〜[hibernate-core-5.2.12.Final.jar：5.2.12.Final] 
  

这并不令我感到惊讶。但我想以某种方式能够在没有整个 Cart 实例的情况下实例化 Item 。理想情况下， Cart ID作为 Item 中的字段就足够了。

编辑

因此，如果我必须使 Cart 在项目中如何使用 data-h2.sql 脚本初始化数据库？有没有SQL方式来表达 Cart 是！

解决方案

您可以将引用完全移回到 Cart 中，并通过删除 mappedBy =cart code> @OneToMany 注释中的code>部分。

如果您仍然希望 cart_id ，您可以在 Cart 中使用java代码来设置它。

当然，所有这些都引出了一个问题：为什么你想要这种非JPA-ish的行为。如果您创建另一个问题，说明您的原始问题/关注，似乎有可能需要了解JPA。

I am experimenting with spring and jpa's ManyToOne in this repo. The relationship between the entities is that one Cart (shopping cart) can have many Items (following this tutorial).

Cart

@Entity
@Table(name="CART")
public class Cart 
{
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "cart_id")
    private long id;


    @OneToMany(mappedBy = "cart")
    private Set<Item> items;

    /* rest of class*/

Item

@Entity
@Table(name="ITEMS")
public class Item
{
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private long id;

    @Column(name="item_name")
    private String itemName;

    @ManyToOne
    @JoinColumn(name = "cart_id", nullable = false)
    private Cart cart; // needs an entire Cart instance :(

     /* rest of class*/

Question

How can I map the Items to the Cart class without making the entire Cart object part of the Item class.

I want to look the Item class to look like this

@Entity
@Table(name="ITEMS")
public class Item
{
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private long id;

    @Column(name="item_name")
    private String itemName;


    @ManyToOne
    @JoinColumn(name = "cart_id", nullable = false)
    private long cartId; // just the ID of Cart, not the entire Cart object

This gives the errors

Caused by: org.hibernate.AnnotationException: @OneToOne or @ManyToOne on com.example.demo.Item.cartId references an unknown entity: long
    at org.hibernate.cfg.ToOneFkSecondPass.doSecondPass(ToOneFkSecondPass.java:97) ~[hibernate-core-5.2.12.Final.jar:5.2.12.Final]
    at org.hibernate.boot.internal.InFlightMetadataCollectorImpl.processEndOfQueue(InFlightMetadataCollectorImpl.java:1752) ~[hibernate-core-5.2.12.Final.jar:5.2.12.Final]
    at org.hibernate.boot.internal.InFlightMetadataCollectorImpl.processFkSecondPassesInOrder(InFlightMetadataCollectorImpl.java:1696) ~[hibernate-core-5.2.12.Final.jar:5.2.12.Final]
    at org.hibernate.boot.internal.InFlightMetadataCollectorImpl.processSecondPasses(InFlightMetadataCollectorImpl.java:1583) ~[hibernate-core-5.2.12.Final.jar:5.2.12.Final]
    at org.hibernate.boot.model.process.spi.MetadataBuildingProcess.complete(MetadataBuildingProcess.java:278) ~[hibernate-core-5.2.12.Final.jar:5.2.12.Final]

which doesn't really surprise me. But I want to be somehow able to instantiate an Item without an entire Cart instance. Ideally, just the Cart ID as a field in Item would suffice.

EDIT

So if I have to make Cart a field in Item how to I initialize the db with a data-h2.sql script? There is no SQL way to express what a Cart is!

解决方案

You can just remove the reference back to the Cart completely and make the complete relationship go just in one direction by dropping the mappedBy="cart" part from the @OneToMany annotation.

If you still want the cart_id in the item you can set it using java code on the Cart.

Of course, all this begs the question: Why do you want this behavior which is very un-JPA-ish. If you create another question stating your original question/concern it seems likely that there is something to learn about JPA.

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