我创建了以下实体来管理持久购物车:
购物车.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ShoppingCart
中的所有ShoppingCartItem
都是在单个查询中按预期检索的,但@ManyToOne private Item item;
字段不在联接中,并且当访问时,会为每个ShoppingCartItem
发出一个单独的查询来获取该字段。
使用EclipseLink,是否有一种方法可以在连接/批量获取ShoppingCartItem
时也获取Item
的连接?如何更改查询和/或代码?
如果您正在使用EclipseLink,您可以查看@BatchFetch和@JoinFetch注释。
虽然带有别名的left join fetch
似乎被忽略了,但我发现了以下查询提示:
Query query = entityManager.createQuery("SELECT c FROM ShoppingCart c WHERE c.id = :id");
query.setHint("eclipselink.left-join-fetch", "c.items.item.brand");
这可能比注释方法更好,因为它可以为单个查询指定。
更新
使用此提示会破坏@OrderBy("creationTimestamp")
,因此ShoppingCartItem
不再按插入顺序返回。这可能是由于EclipseLink中的一个错误造成的,但我认为这并没有造成太大的伤害,因为我实际上只需要在向用户显示购物车时订购物品,而不是在用户登录时,匿名购物车中的物品必须转移到用户购物车。