java - 加入获取嵌套在 @OneToMany 中的 @ManyToOne

Question

我创建了以下实体来管理持久购物车:

ShoppingCart.java:

@Entity
public class ShoppingCart {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @PrivateOwned
    @OneToMany(mappedBy = "cart", cascade = CascadeType.ALL)
    @OrderBy("creationTimestamp")
    private List<ShoppingCartItem> items;

    public ShoppingCart() {}

    // Getters and setters...
}

ShoppingCartItem.java:

@Entity
@IdClass(ShoppingCartItemId.class)
public class ShoppingCartItem {
    @Id
    @ManyToOne
    private Item item;

    @Id
    @ManyToOne
    private ShoppingCart cart;

    private int quantity;

    @Column(precision = 17, scale = 2)
    private BigDecimal price;

    @Temporal(TemporalType.TIMESTAMP)
    private Date creationTimestamp;

    protected ShoppingCartItem() {}

    @PrePersist
    protected void prePersist() {
        creationTimestamp = new Date();
    }

    public ShoppingCartItem(ShoppingCart cart, Item item, int quantity) {
        this.cart = cart;
        this.item = item;
        this.quantity = quantity;
        this.price = item.getPrice();
    }

    // Getters and setters...
}

项目.java:

@Entity
public class Item {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne
    private Brand brand;

    private String model;
    private String variant;
    private String description;

    @Column(precision = 17, scale = 2)
    private BigDecimal price;

    private int availability;

    protected Item() {}

    // Constructors, getters and setters...
}

当我发出以下 JPQL 查询时:

SELECT c FROM ShoppingCart c JOIN FETCH c.items WHERE c.id = :id

我注意到同一个 ShoppingCart 中的所有 ShoppingCartItem 都按预期在单个查询中检索，但 @ManyToOne 私有(private) Item 项； 字段不在连接中，并且对每个 ShoppingCartItem 发出单独的查询，以获取该字段 访问时 。

使用 EclipseLink，有没有办法在加入/批量获取 ShoppingCartItem 时也获取 Item 的加入？如何更改查询和/或代码？

Answer 1

如果您使用 EclipseLink，您可以查看 @BatchFetch和 @JoinFetch注释。