@Transactional 它本身应该反映对数据库中的实体所做的更改。我正在创建一个应用程序，客户机可以在其中创建一个像这样的car实体（ update 方法是以后用的放，不注意了 brand 属性）：

@Entity
@Table(name = "cars")
public class Car {
    @Id
    @GeneratedValue(generator = "inc")
    @GenericGenerator(name = "inc", strategy = "increment")
    private int id;
    @NotBlank(message = "car name`s must be not empty")
    private String name;
    private LocalDateTime productionYear;
    private boolean tested;

    public Car() {
    }

    public Car(@NotBlank(message = "car name`s must be not empty") String name, LocalDateTime productionYear) {
        this.name = name;
        this.productionYear = productionYear;
    }

    @ManyToOne
    @JoinColumn(name = "brand_id")
    private Brand brand;

    public int getId() {
        return id;
    }

    void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public LocalDateTime getProductionYear() {
        return productionYear;
    }

    public void setProductionYear(LocalDateTime productionYear) {
        this.productionYear = productionYear;
    }

    public boolean isTested() {
        return tested;
    }

    public void setTested(boolean tested) {
        this.tested = tested;
    }

    public Brand getBrand() {
        return brand;
    }

    void setBrand(Brand brand) {
        this.brand = brand;
    }

    public Car update(final Car source) {
        this.productionYear = source.productionYear;
        this.brand = source.brand;
        this.tested = source.tested;
        this.name = source.name;
        return this;
    }
}

在我的应用程序中，客户机可以使用put方法创建一辆新车或更新一辆现有的车。
我的控制器：

@RestController
public class CarController {
    private Logger logger = LoggerFactory.getLogger(CarController.class);
    private CarRepository repository;

    public CarController(CarRepository repository) {
        this.repository = repository;
    }

    //The client can create a new resource or update an existing one via PUT
    @Transactional
    @PutMapping("/cars/{id}")
    ResponseEntity<?> updateCar(@PathVariable int id, @Valid @RequestBody Car source) {
        //update
        if(repository.existsById(id)) {
            repository.findById(id).ifPresent(car -> {
                car.update(source); //it doesn`t work
                //Snippet below works
                //var updated = car.update(source);
                //repository.save(updated);
            });
            return ResponseEntity.noContent().build();
        }
        //create
        else {
            var result = repository.save(source);
            return ResponseEntity.created(URI.create("/" + id)).body(result);
        }
    }
}

当我创造一辆新车时，它是有效的。但是，如代码中所述，当没有save方法时，尽管我得到了状态204（无内容），但实体没有更改。如果有save方法，它就可以正常工作。你知道为什么会这样吗？
其中一个用户向我请求 Brand 实体。我没有创造任何 Brand 但本质上 Car 可以属于特定的 Brand 在我的应用程序中。到目前为止，还没有 Car 属于任何 Brand . 这是一个实体：

@Entity
@Table(name = "brands")
public class Brand {
    @Id
    @GeneratedValue(generator = "i")
    @GenericGenerator(name = "i", strategy = "increment")
    private int id;
    @NotBlank(message = "brand name`s must be not empty")
    private String name;
    private LocalDateTime productionBrandYear;

    @OneToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL, mappedBy = "brand")
    private Set<Car> cars;

    @ManyToOne
    @JoinColumn(name = "factory_id")
    private Factory factory;

    public Brand() {
    }

    public int getId() {
        return id;
    }

    void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public LocalDateTime getProductionBrandYear() {
        return productionBrandYear;
    }

    public void setProductionBrandYear(LocalDateTime productionBrandYear) {
        this.productionBrandYear = productionBrandYear;
    }

    public Set<Car> getCars() {
        return cars;
    }

    public void setCars(Set<Car> cars) {
        this.cars = cars;
    }

    public Factory getFactory() {
        return factory;
    }

    public void setFactory(Factory factory) {
        this.factory = factory;
    }
}