在javaspring引导中减少另一个表中列的值

我为应用程序构建了后端进程,使用 弹簧靴,maven 我有3个域(客户、产品、订单),这是表的列。

        A. Customer:  1. id,
                      2. name,
                      3. address,
                      4. email,
                      5. phone.

        B. Product :  1. id,
                      2. name,
                      3. stock,
                      4. price.

        C. Order   :  1. id,
                      2. customer_id,
                      3. product_id,
                      4. quantity.

秩序 product.stock 和……一样多 order.quantity .

我应该如何处理我的代码才能做到这一点?

这是我的 订单.java

package com.learn.ecommerce.domain;

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;

import javax.persistence.*;
import javax.validation.constraints.Min;
import javax.validation.constraints.NotNull;
import java.io.Serializable;
import java.util.Objects;

/**
 * A order model.
 */
@Entity
@Table(name = "orders")
public class Order extends AbstractAuditingEntity implements Serializable {

private static final long serialVersionUID = 1L;

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

@ManyToOne
@JsonIgnoreProperties("")
private Customer customer;

@ManyToOne
@JsonIgnoreProperties("")
private Product product;

@NotNull
@Min(0)
@Column(nullable = false)
private Integer quantity;

public Long getId() {
    return id;
}

public void setId(Long id) {
    this.id = id;
}

public Customer getCustomer() {
    return customer;
}

public void setCustomer(Customer customer) {
    this.customer = customer;
}

public Product getProduct() {
    return product;
}

public void setProduct(Product product) {
    this.product = product;
}

public Integer getQuantity() {
    return quantity;
}

public void setQuantity(Integer quantity) {
    this.quantity = quantity;
}

@Override
public boolean equals(Object o) {
    if (this == o)
        return true;

    if (o == null || getClass() != o.getClass())
        return false;

    Order order = (Order) o;

    return !(order.getId() == null || getId() == null) && Objects.equals(getId(), order.getId());
}

@Override
public int hashCode() {
    return Objects.hashCode(getId());
}

@Override
public String toString() {
    return "Order{" +
            "id=" + id +
            ", customer=" + customer +
            ", product=" + product +
            ", quantity=" + quantity +
            '}';
}

这是我的

package com.kevin.ecommerce.service.impl;

import com.kevin.ecommerce.domain.Order;
import com.kevin.ecommerce.repository.OrderRepository;
import com.kevin.ecommerce.service.OrderService;
import com.kevin.ecommerce.service.dto.OrderDTO;
import com.kevin.ecommerce.service.mapper.OrderMapper;
import lombok.extern.slf4j.Slf4j;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.data.domain.Page;
import org.springframework.data.domain.Pageable;
import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;

import java.util.LinkedList;
import java.util.List;
import java.util.Optional;
import java.util.stream.Collectors;

@Slf4j
@Service
@Transactional
public class OrderServiceImpl implements OrderService {

@Autowired
private OrderRepository orderRepository;

/**
 * Save a order
 *
 * @param orderDTO the entity to save
 * @return the persisted entity
 */
@Override
public OrderDTO save(OrderDTO orderDTO) {
    log.debug("Request to save Order : {}", orderDTO);
    Order order = OrderMapper.INSTANCE.toEntity(orderDTO);
    order = orderRepository.save(order);
    return OrderMapper.INSTANCE.toDto(order);
}

/**
 * Get all of orders
 *
 * @return the list of entities
 */
@Override
@Transactional(readOnly = true)
public List<OrderDTO> findAll() {
    log.debug("Request to get all Orders");
    return orderRepository.findAll().stream()
            .map(OrderMapper.INSTANCE::toDto)
            .collect(Collectors.toCollection(LinkedList::new));
}

/**
 * Get specific order
 *
 * @param id the id of entity
 * @return the entity
 */
@Override
@Transactional(readOnly = true)
public Optional<OrderDTO> findOne(Long id) {
    log.debug("Request get Order : {}", id);
    return orderRepository.findById(id)
            .map(OrderMapper.INSTANCE::toDto);
}

/**
 * Delete specific order
 *
 * @param id the id of entity
 */
@Override
public void delete(Long id) {
    log.debug("Request to delete Order : {}", id);
    orderRepository.deleteById(id);
}

/**
 * Get all of orders by page
 * @param pageable
 * @return
 */
@Override
@Transactional(readOnly = true)
public Page<OrderDTO> findAll(Pageable pageable) {
    log.debug("pageable");
    return orderRepository.findAll(pageable).map(OrderMapper.INSTANCE::toDto);
}

}

OrderRepository.java文件

package com.nostratech.ecommerce.repository;

import com.nostratech.ecommerce.domain.Order;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

@Repository
public interface OrderRepository extends JpaRepository<Order, Long> {
}

产品存储库.java

package com.nostratech.ecommerce.repository;

import com.nostratech.ecommerce.domain.Product;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

@Repository
public interface ProductRepository extends JpaRepository<Product, Long> {
}

public OrderDTO save ,但我是java后端的新手,不知道该怎么做。

谢谢