JPA 实体，如何连接表

Question

我有三张表

CREATE TABLE "ingredient" (
  "id" INTEGER GENERATED BY DEFAULT AS IDENTITY(START WITH 1, INCREMENT BY 1) PRIMARY KEY,
  "ingredient" VARCHAR(50) NOT NULL
);


CREATE TABLE "pizza" (
  "id" INTEGER GENERATED BY DEFAULT AS IDENTITY(START WITH 1, INCREMENT BY 1) PRIMARY KEY,
  "pizza" VARCHAR(50) NOT NULL
);


CREATE TABLE "pizza_structure" (
  "pizza_id" INT NOT NULL,
  "ingredient_id" INT NOT NULL,
  "amount" INT NOT NULL
);

如何加入它们，将 Pizzas 结构作为 Map

@Entity
@Table(name = "ingredient")
public class Ingredient{

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

    private String name;

    public Ingredient() {
    }
}


@Entity
@Table(name = "pizza")
public class Pizza {

    @Id
    @GeneratedValue
    private Long id;
    private String name;
    @OneToMany ????
    private Map<Ingredient, Integer> pizzaStructure;

    public Pizza() {
    }

    public Pizza(String name, Map<Long, Integer> pizzaStructure) {
        this.name = name;
        this.pizzaStructure = pizzaStructure;
    }
}

我需要创建@Embeddable class PizzaStructure，如果需要，什么时候使用它？

现在我收到一个错误 init 方法调用失败；嵌套异常是 org.hibernate.AnnotationException：使用 @OneToMany 或 @ManyToMany 定位未映射的 class:

Answer 1

你在比萨饼和配料之间有一个多对多的关系，并且在你的关系中有一个额外的列。我在这里发现了一个类似的问题：JPA 2.0 many-to-many with extra column （我会评论，但我没有足够的声誉。）

Answer 2

how to join them, to get Pizzas structure as a Map

看起来像这样：

@ElementCollection
@CollectionTable(name = "pizza_structure", joinColumns = {@JoinColumn(name = "pizza_id")})
@Column(name = "amount")
@MapKeyJoinColumn(name = "ingredient_id")
private Map<Ingredient, Integer> pizzaStructure;

do I need to create @Embeddable class PizzaStructure

没有

更多信息在这里：Hibernate User Guide - Maps。

注意 table pizza_structure 应该有 pizza 和 ingredient table 的外键以及 pizza_id 和 pizza_id 的唯一约束ingredient_id，像这样（这是 postgresql 方言）：

create table pizza_structure
(
  pizza_id ... constraint fk_structure_pizza references pizza,
  ingredient_id ... constraint fk_structure_ingredient references ingredient,
  amount ...,
  constraint pizza_structure_pkey primary key (pizza_id, ingredient_id)
);

JPA 实体，如何连接表

JPA Entitties, how to join tables

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