Hibernate中扩展类的查询属性

Question

我有四个类，一个代表一个产品，另一个代表这个产品的一个属性，另外两个保存这个属性的值，因为它可以是字符串或整数，都有 getter 和 setter（这里省略）。

产品.class

@Entity
@Table(name = "product")
public class Product {

    @Id
    @GeneratedValue(strategy = IDENTITY)
    @Column(name = "id")
    private Integer id;

    @Column(name = "name")
    private String name;

    @OneToMany(mappedBy = "product", cascade = CascadeType.MERGE, fetch = FetchType.EAGER, orphanRemoval = true)
    private Set<Attribute> attributes = new HashSet<>(0);

    [...]
}

属性类

@Entity
@Table(name = "attribute")
@Inheritance(strategy = InheritanceType.JOINED)
@DiscriminatorColumn(name = "type", discriminatorType = DiscriminatorType.INTEGER)
public class Attribute {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id", unique = true, nullable = false)
    private Long id;

    @ManyToOne
    @JoinColumn(name = "product_id")
    private Product product;

    @ManyToOne
    @JoinColumn(name = "custom_attribute_id")
    private CustomAttribute customAttribute;

    @Column(name = "type")
    private Integer type;

    [...]
}

AttributeString.class

@Entity
@Table(name = "attribute_string")
@DiscriminatorValue("1")
public class AttributeString extends Attribute {

    @Column(name = "value")
    private String value;

    [...]
}

AttributeInteger.class

@Entity
@Table(name = "attribute_integer")
@DiscriminatorValue("2")
public class AttributeInteger extends Attribute {

    @Column(name = "value")
    private Integer value;

    [...]
}

我想使用 Hibernate Criteria 创建一个投影查询来计算产品具有的不同属性和值的数量，如下所示：

// Projection query to list the summary of attribute
Session session = (Session) em.getEntityManagerFactory().createEntityManager()
            .getDelegate();
Criteria criteria = session.createCriteria(Product.class);
criteria.creatAlias("attributes", "attribute");
criteria.creatAlias("attribute.customAttribute", "customAttribute");
Projection projectionSummary = Projections.projectionList()
        .add(Projections.groupProperty("customAttribute.id"))
        .add(Projections.groupProperty("attribute.value"))
        .add(Projections.count("id"));
List<Object[]> items = criteria.setProjection(projectionSummary).list();

但这会引发错误org.hibernate.QueryException: could not resolve property: value of com.example.entity.Attribute

Answer 1

正如你的例外所说

无法解析属性：com.example.entity.Attribute 的值

在Attribute.class，getter 和setter 方法有问题。它可能不遵循约定。

如果你使用任何IDE like eclipse，那么尝试使用它来创建getter setter方法，这将帮助你更多地遵循约定。

为id, product, customAttribute and type 创建getter 和setter。

以id为例：

public Long getId() {
    return id;
}

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

更新：

我认为attributes 将是attribute。请检查我是否错了。

criteria.creatAlias("attributes", "attribute");

使用以下代码

criteria.creatAlias("attribute", "attribute");