Entity Manager returns duplicate copies of a @OneToMany related Entity

Question

I am developing an application to help my office track and manage reviews. In the application, I am using JPA 2.0 with Hibernate 3.6.3 as my underlying provider. I am also using spring to inject the persistence context into my DAO. I have built the domain such that there is a Review, a Participant and a Role Entity.

The problem I am having is when I get a Review from the Entity Manager there are duplicate copies of the same Participant(i.e. same ID) in the list of participants if the Participant has more than one Role. I have also found the number of duplicates is directly related to the number Roles (i.e. if the a Participant has 3 Roles then the Participant occurs 3 times in the Review's list of participants)

I have used straight Hibernate before but this is my first time using JPA so I am sure I have configured something wrong. I just don't know what it is.

Here is the code:

Review:

@Entity
public class Review extends BaseModel{

@ManyToOne(cascade=CascadeType.ALL, fetch=FetchType.EAGER, optional=false)
private Item item;

@Column(name="ISNEW", nullable=false)
private boolean isNew;

@Enumerated(EnumType.STRING)
@Column(name="STATUS", nullable=false)
private ReviewStatus status;

@Enumerated(EnumType.STRING)
@Column(name="PHASE", nullable=false)
private Phase phase;

@Enumerated(EnumType.STRING)
@Column(name="REVIEW_TYPE", nullable=false)
private ReviewType reviewType;

@OneToMany( cascade=CascadeType.ALL, fetch=FetchType.EAGER)
private List<Participant> participants;

@OneToMany(cascade=CascadeType.ALL)
private List<Defect> defects;

@Column(name="START_DATE", nullable=false)
private Date startDate;

@Column(name="MEETING_DATE", nullable=false)
private Date meetingDate;

@Column(name="FINISH_DATE")
private Date finishDate;

@Column(name="DURATION", nullable=false)
private Double duration;

public Item getItem()
{
    return item;
}
public void setItem(Item item)
{
    this.item = item;
}   
public List<Participant> getParticipants() {
    return participants;
}
public void setParticipants(List<Participant> participants) {
    this.participants = participants;
}
public List<Defect> getDefects() {
    return defects;
}
public void setDefects(List<Defect> defects) {
    this.defects = defects;
}   
public boolean isNew() {
    return isNew;
}
public void setNew(boolean isNew) {
    this.isNew = isNew;
}
public Phase getPhase() {
    return phase;
}
public void setPhase(Phase phase) {
    this.phase = phase;
}
public Double getDuration() {
    return duration;
}
public void setDuration(Double duration) {
    this.duration = duration;
}
public ReviewStatus getStatus() {
    return status;
}
public void setStatus(ReviewStatus status) {
    this.status = status;
}
public Date getStartDate() {
    return startDate;
}
public void setStartDate(Date startDate) {
    this.startDate = startDate;
}
public Date getMeetingDate() {
    return meetingDate;
}
public void setMeetingDate(Date meetingDate) {
    this.meetingDate = meetingDate;
}
public Date getFinishDate() {
    return finishDate;
}
public void setFinishDate(Date finishDate) {
    this.finishDate = finishDate;
}
public ReviewType getReviewType()
{
    return reviewType;
}
public void setReviewType(ReviewType reviewType)
{
    this.reviewType = reviewType;
}

}

Participant:

@Entity
public class Participant extends BaseModel{


        private Double inspectionTime;

        @ManyToOne(cascade=CascadeType.ALL, optional=false)
        private Person person;

        @ManyToMany(cascade=CascadeType.ALL, fetch=FetchType.EAGER )
        private Set<Role> roles;

        public Double getInspectionTime() {
            return inspectionTime;
        }
        public void setInspectionTime(Double preInspectionTime) {
            this.inspectionTime = preInspectionTime;
        }
        public Person getPerson() {
            return person;
        }
        public void setPerson(Person person) {
            this.person = person;
        }
        public Set<Role> getRoles() {
            return roles;
        }
        public void setRoles(Set<Role> roles) {
            this.roles = roles;
        }

}

Role:

@Entity
public class Role extends BaseModel{
            @Column(nullable=false, unique=true)
            private String name;
            private String responsiblity;

            public String getName() {
                return name;
            }
            public void setName(String name) {
                this.name = name;
            }
            public String getResponsiblity() {
                return responsiblity;
            }
            public void setResponsiblity(String responsiblity) {
                this.responsiblity = responsiblity;
            }

}

Base:

@MappedSuperclass
public abstract class BaseModel {

    @Id
    @GeneratedValue
    private Long id;

    public Long getId() {
        return id;
    }

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

}

DataAccessObject which uses the entityManager:

@Transactional(readOnly = false, propagation = Propagation.REQUIRED)
public class ReviewDaoJpaImpl implements ReviewDao
{


    @PersistenceContext
    private EntityManager em;

    public Review getReviewById(Long id)
    {
        return em.find(Review.class, id);
    }
}

Call which receives duplicates:

    Review review = reviewDao.getReviewById(1776L);
    List<Participant> participants = review.getParticipants();
    for (Participant participant : participants)
    {
        System.out.println(participant.getPerson().getName());
    }

gandonies which upvotes the answer -> before read it and then draw a conclusion! This is a helpful for 10 most known issue on Hibernate(ORM) - https://www.thoughts-on-java.org/common-hibernate-mistakes-cripple-performance/ Additionall info for massaging result set -> https://howtoprogramwithjava.com/how-to-fix-duplicate-data-from-hibernate-queries/ — Musa, Jul 03 '18 at 13:36

Ralph · Accepted Answer · 2011-04-21T07:23:33.360

12

The problem is called (N+1 Problem) and is related to the eager fetch of Roles in Participant.

The easiest way the handle it would be replacing the eager fetch by lazy loading.

edited Apr 21 '11 at 07:23

answered Apr 21 '11 at 07:15

Ralph

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That was it. I am embarrassed that N+1 did not turn up on any of my searches about the problem. I guess I wasn't using the right keywords. Thanks for your help. I knew I was doing something wrong. – Scott Apr 21 '11 at 21:21
Can you please explain more about the N+1? – Dejell Jun 20 '13 at 22:01
@Odelya Google for "n+1 problem in hibernate" first hit is: http://stackoverflow.com/questions/97197/what-is-the-n1-selects-issue – Ralph Jun 21 '13 at 12:37
3

Can also use @Fetch(FetchMode.SUBSELECT) instead of making it lazy. – mR_fr0g Jun 18 '15 at 11:08

score 4 · Answer 2 · answered Dec 10 '13 at 07:47

Complete Reference to Hibernate Documentation

Just put Criteria.DISTINCT_ROOT_ENTITY it will internally create a Set and duplicates are automatically removed.

List result = session.createCriteria(Order.class)  
                        .setFetchMode("lineItems", FetchMode.JOIN)  
                        .setResultTransformer(Criteria.DISTINCT_ROOT_ENTITY)  
                        .list();

Entity Manager returns duplicate copies of a @OneToMany related Entity

2 Answers2