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我在 java ee6 环境中使用 JPA2 和 hibernate 作为提供程序。

我有一个一对多的关系，当让一侧的所有行显示在 JSF 页面中时，我想显示一些多侧的属性，**但不是全部 **，甚至避免加载这些属性时填充许多侧面实体，我不知道该怎么做。

这是一个片段

我想获取表 A 的所有行并将它们显示在 jsf 表中，当显示表 A 的行时，我还想在同一个 jsf 表中显示表 B 中的一些属性，理想情况下甚至不检索/存储表中未使用的属性乙

TBL_A
________________
int     ID
varchar FIRSTNAME
varchar LASTNAME

第二个是

TBL_B
___________
int      ID
varchar  VERSION  // display on jsf page
varchar  XML_DATA //large, don't want to load when getting results for A/B join
int      A_ID

和我的 JPA2 关联类是这样的

@Entity
@Table(name = "TBL_A")
@NamedQueries // not sure what sql to create to accomplish this?
({ @NamedQuery(name = "A_and_B.findAll", query = "SELECT a FROM EntityA a") })

public class EntityA implements Serializable 
{    
   @Id
   @GeneratedValue(strategy=GenerationType.IDENTITY)
   @NotNull
   @Column(name = "ID", nullable = false)
   private Integer id;
.

@Column(name = "FIRSTNAME")
private String firstname;
.
@Column(name = "LASTNAME")
private String lastname;
.
@OneToMany(mappedBy = "entityA", fetch = FetchType.EAGER)
private List<EntityB> entityBList;

我的 TBL_B 关联实体是

@Entity
@Table(name = "TBL_B")  
public class EntityB implements Serializable 
{ 
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
@NotNull
@Column(name = "ID", nullable = false)
private Integer id;
.        
@Column(name = "VERSION")
private String version;
.
@Column(name = "XML_DATA")
private String xmlData;
.
@JoinColumn(name = "A_ID", referencedColumnName = "ID")
@ManyToOne
private EntityA entityA;

I have a JSF page that displays a datatable/list of TBL_A entries and also the VERSION column from TBL_B, when getting all the rows for TBL_A.

I had to use the fetch type of EAGER to avoid hibernate lazy instatiation error to do that. hope thats the correct way to do it.

when loading all the TBL_B entities I guess I don't mind eagerly loading all TBL_B rows in entityBList, but I don't want the list of EntityB beans to have thier xmlData property loaded cause it is significantly large.

can someone explain how I can do this?

how can i make this happen with my named query

public List<EntityA> findAll_A_Entities() 
{
   return em.createNamedQuery("A_and_B.findAll", EntityA.class).getResultList();
}
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1 回答 1

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One solution is to make the property lazy - but that requires additional build configuration - refer to the doc.

The other option is to move the xmlData Field to a derived class and use hibernate inheritance support.

If you were querying EntityB directly you can use projection query to selected only the required fields. When it is in a collection association that is not possible.

于 2011-12-01T03:53:04.637 回答