javax.persistence.NonUniqueResultException：查询未返回唯一结果

Question

我正在使用带有 Spring-data-jpa 的 Spring Boot

我有两个实体 Category 和 Course。它们通过@ManyToMany 关联。

  public class Course {
        @Id
        @GeneratedValue(generator = "inc")
        @GenericGenerator(name = "inc", strategy = "increment")
        private int courseId;
        @NotBlank(message = "Add the course's title!")
        private String title;
    
        @ManyToMany(mappedBy = "courses", fetch = FetchType.EAGER)
        private Set<Category> categories;
}

public class Category {
    @Id
    @GeneratedValue(generator = "inc")
    @GenericGenerator(name = "inc", strategy = "increment")
    private int categoryId;
    @NotBlank(message = "category's name must be not empty!")
    private String name;

    @ManyToMany(cascade = CascadeType.PERSIST)
    @JoinTable(
            name="course_category",
            joinColumns = @JoinColumn(name = "category_id"),
            inverseJoinColumns = @JoinColumn(name = "course_id")
    )
    private Set<Course> courses;

当我下载一个类别时，我也想下载它的所有课程（没有fetchType.EAGER，因为我不想在下载所有类别的同时获取课程）。这是我在CategoryRepository (JpaRepository) 中覆盖的查询：

    @Override
    @Query("SELECT cat FROM Category cat JOIN FETCH cat.courses")
    Optional<Category> findById(Integer id); 

因为我想从两个表中发送我的客户数据，所以我创建了一个 DTO：

public class CategoryAndCourses {
    private Category category;
    private List<Course> courses;

    public CategoryAndCourses(Category category, List<Course> courses) {
        this.category = category;
        this.courses = courses;
    }

还有一个地方，我实际上是通过 DTO 发送课程和类别：

public CategoryAndCourses getCourses(int id) {
        var category = repository.findById(id)
                .orElseThrow(() -> new IllegalArgumentException("No category with given id"));
            var result = new CategoryAndCourses(category, new ArrayList<>(category.getCourses()));
        return result;
    }

使用这样的代码我得到这个错误： javax.persistence.NonUniqueResultException: query did not return a unique result: 8 如何解决？

Answer 1

错误很明显：您有一个查询返回超过 1 个结果，并且您的代码被定义为期望单个结果。

您是否在以下查询的id 上缺少WHERE？

@Query("SELECT cat FROM Category cat JOIN FETCH cat.courses")
Optional<Category> findById(Integer id);