Child Entities Count in unidirectional @OneToMany mapping in hibernate

Question

I am working on a project where two entities have unidirectional @OneToMany mapping. When I tried to load all child entities for particular parent id with this query -

select p.childEntities from Parent p where p.id =:parentId

It works fine. Also note that here I am using hibernate api for pagination so I get only 10, 25 ... records. At many points I only need count of entities. Now I am trying to load only count of all child entities with this query-

select count(p.childEntities) from Parent p where p.id =:parentId

which fails with ORACLE error code - ORA-00936: missing expression

My situation(Low authority for this project)- I can not change the entity mappings into bidirectional. And No use of native SQL. Also I think that getting all list using -

" select p.childEntities from Parent p where p.id =:parentId "

and then just getting size() for count is performance costly.

A glimpse of mappings in project- Parent Class-

 @Entity
@Table(name = "PARENT")
public class Parent implements Serializable{

    private static final long             serialVersionUID     = 2232715856164345328L;

    private Long   id;

    private String first;

    private String second;

    private String third;

    @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER)
    @JoinColumn(name = "parent_id")
    private List<Child> childEntities;


    public Long getId() {
        return id;
    }

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

    public String getFirst() {
        return first;
    }

    public void setFirst(String first) {
        this.first = first;
    }

    public String getSecond() {
        return second;
    }

    public void setSecond(String second) {
        this.second = second;
    }

    public String getThird() {
        return third;
    }

    public void setThird(String third) {
        this.third = third;
    }

    public List<Child> getChildEntities() {
        return childEntities;
    }

    public void setChildEntities(List<Child> childEntities) {
        this.childEntities = childEntities;
    }

    public static long getSerialversionuid() {
        return serialVersionUID;
    }

}

And Child Class-

@Entity
@Table(name = "Child")
public class Child {

    private Long   id;

    private Integer number;

    private String prop1;

    private String prop2;

    public Long getId() {
        return id;
    }

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

    public Integer getNumber() {
        return number;
    }

    public void setNumber(Integer number) {
        this.number = number;
    }

    public String getProp1() {
        return prop1;
    }

    public void setProp1(String prop1) {
        this.prop1 = prop1;
    }

    public String getProp2() {
        return prop2;
    }

    public void setProp2(String prop2) {
        this.prop2 = prop2;
    }
}

What all choices I am left with?

Answer 1

您需要了解联接（在SQL，BTW中也将使用联接）：

select count(c.id) from Parent p join p.childEntities c where p.id = :parentId