使用 CriteriaBuilder 的 JPA 左外连接导致错误：部分对象查询不允许维护缓存或被编辑

Question

I have the following entity relationships

Product

@Entity
@Table(name="PRODUCTS")
public class Product

@Id
@Column(name="product_id")
private long productId;

@ManyToOne
@JoinColumn(name="EMP_NUMBER")
private Employee employee3; 
....
....

Employee

@Entity
@Table(name="EMPLOYEES")
public class Employee

@Id
@Column(name="EMP_NUMBER")
private String empNumber;

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

@OneToMany(mappedBy="employee3")
private List<Product> Product3;

....
....

In DAOImpl class I have the following

CriteriaBuilder cb = getEntityManager().getCriteriaBuilder();
CriteriaQuery<Product> cq = 
            cb.createQuery(Product.class);
Root<Product> c = cq.from(Product.class);
Join<Product, Employee> join = 
           c.join(Product_.employee3, JoinType.LEFT);
cq.multiselect(c.get(Product_.productId),        
           c.get(Product_.employee3));

However when I execute, I am getting the following errors

org.eclipse.persistence.exceptions.QueryException Exception Description: Partial object queries are not allowed to maintain the cache or be edited.
You must use dontMaintainCache(). Query: ReadAllQuery(referenceClass=Product ) at org.eclipse.persistence.exceptions.QueryException.cannotCachePartialObjects (QueryException.java:388) at org.eclipse.persistence.queries.ObjectLevelReadQuery.prepareQuery (ObjectLevelReadQuery.java:2160)

What I am trying to achieve is to generate the following SQL

SELECT p.product_id,
          emp.emp_name
     FROM products p
          LEFT OUTER JOIN employees emp
             ON (p.emp_number = emp.emp_number)

How can I do this and get rid of errors?

Answer 1

I have resolved the issue by adding the below mentioned line, might be useful to others if they encounter same problem what been mentioned in question.

((org.eclipse.persistence.jpa.JpaQuery)query).getDatabaseQuery().dontMaintainCache();

Another better option is provided here

Thanks

Answer 2

According to this , there are two ways to create a criteria query createQuery(class) and createQuery() without the parameter. The first method creates a CriteriaQuery cast the result to class .

This means to resolve the problem, you can change CriteriaQuery<Product> cq = cb.createQuery(Product.class); to this one CriteriaQuery<Product> cq = cb.createQuery(); .

And then use an Iterator to retrieve the result.

Iterator products = query.getResultList().iterator();

Answer 3

In my case and according to this: https://bugs.eclipse.org/bugs/show_bug.cgi?id=303205 The issue happens when you apply a multiselect and don't have a constructor with the selected parameters in the representation object.

For instance if you have

Select empNumber,employeeName from Employee

Then in your entity or representation object you need a constructor:

public Employee(int empNumber, StringemployeeName ){...}

Be careful since the order of the parameters matter.