它是我的实体字段。结构类似->
购物车[cartid=1，userid=1，ordeitem=[orderitem[pid=12，qty=3，cart=null]，orderitem[pid=4，qty=5，cart=null]]

@Entity
@Table(name = "cart")
public class Cart {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long cartId;
    @Column(nullable = false, unique = false)
    private Long userId;
    @ElementCollection
//  @OneToMany(targetEntity=OrderItem.class, mappedBy="pid", fetch=FetchType.EAGER)
    @OneToMany(fetch = FetchType.EAGER, mappedBy = "pid")
    private List<OrderItem> ordeItem;
    public Cart(){}

    public Cart(Long cartId, Long userId, List<OrderItem> ordeItem) {
        super();
        this.cartId = cartId;
        this.userId = userId;
        this.ordeItem = ordeItem;
    }

    @Override
    public String toString() {
        return "Cart [cartId=" + cartId + ", userId=" + userId + ", ordeItem=" + ordeItem + "]";
    }

    public Long getCartId() {
        return cartId;
    }

    public void setCartId(Long cartId) {
        this.cartId = cartId;
    }

    public Long getUserId() {
        return userId;
    }

    public void setUserId(Long userId) {
        this.userId = userId;
    }

    public List<OrderItem> getOrdeItem() {
        return ordeItem;
    }

    public void setOrdeItem(List<OrderItem> ordeItem) {
        this.ordeItem = ordeItem;
    }

    @Entity
    static class OrderItem {
        @Id
        private Long pid;
        private Long qty;

        @ManyToOne(fetch = FetchType.EAGER, targetEntity = Cart.class)
        @JoinColumn(name = "cart_id", nullable = false)
        private Cart cart;

        public OrderItem(Long pid, Long qty, Cart cart) {
            super();
            this.pid = pid;
            this.qty = qty;
            this.cart = cart;
        }

        public Long getPid() {
            return pid;
        }

        public void setPid(Long pid) {
            this.pid = pid;
        }

        public Long getQty() {
            return qty;
        }

        public void setQty(Long qty) {
            this.qty = qty;
        }

        public Cart getCart() {
            return cart;
        }

        public void setCart(Cart cart) {
            this.cart = cart;
        }

        @Override
        public String toString() {
            return "OrderItem [pid=" + pid + ", qty=" + qty + ", cart=" + cart + "]";
        }

    }

}

通过 Postman 传递下面的数据

{

    "userId": 1,
    "ordeItem": [
        {
            "pid": 12,
            "qty": 3,
        },
        {
            "pid": 4,
            "qty": 5,
        }
    ]
}

正在尝试将其保存到mysql数据库中。但在使用中保存时出现上述错误。
在服务中，我正在尝试：

public Cart addCart(Cart cart) {
        System.out.println(cart);
        cartDao.save(cart);
        return cart;
    }

但不幸的是，cartid得到了null值，这就是为什么会出现问题的原因。但找不到实际问题。