java - hibernate insert to a collection causes a delete then all the items in the collection to be inserted again

Question

I have a many to may relationship CohortGroup and Employee. Any time I insert an Employee into the CohortGroup hibernate deletes the group from the resolution table and inserts all the members again, plus the new one. Why not just add the new one?

The annotation in the Group:

@ManyToMany(cascade = { PERSIST, MERGE, REFRESH })
@JoinTable(name="MYSITE_RES_COHORT_GROUP_STAFF",
joinColumns={@JoinColumn(name="COHORT_GROUPID")},
inverseJoinColumns={@JoinColumn(name="USERID")})
public List<Employee> getMembers(){
  return members;
}

The other side in the Employee

@ManyToMany(mappedBy="members",cascade = { PERSIST, MERGE, REFRESH } )
public List<CohortGroup> getMemberGroups(){
  return memberGroups;
}

Code snipit

Employee emp = edao.findByID(cohortId);
CohortGroup group = cgdao.findByID(Long.decode(groupId));
group.getMembers().add(emp);
cgdao.persist(group);

below is the sql reported in the log

delete from swas.MYSITE_RES_COHORT_GROUP_STAFF where COHORT_GROUPID=?
insert into swas.MYSITE_RES_COHORT_GROUP_STAFF (COHORT_GROUPID, USERID) values (?, ?)
insert into swas.MYSITE_RES_COHORT_GROUP_STAFF (COHORT_GROUPID, USERID) values (?, ?)
insert into swas.MYSITE_RES_COHORT_GROUP_STAFF (COHORT_GROUPID, USERID) values (?, ?)
insert into swas.MYSITE_RES_COHORT_GROUP_STAFF (COHORT_GROUPID, USERID) values (?, ?)
insert into swas.MYSITE_RES_COHORT_GROUP_STAFF (COHORT_GROUPID, USERID) values (?, ?)
insert into swas.MYSITE_RES_COHORT_GROUP_STAFF (COHORT_GROUPID, USERID) values (?, ?)

This seams really inefficient and is causing some issues. If sevral requests are made to add an employee to the group then some get over written.

Seams like equals and hashCode might be a reason for this. Below are the implementation for these methods. Any red flags?

CohortGroup

    @Override
public int hashCode() {
    final int prime = 31;
    int result = getName().hashCode();
    result = prime * result + ((emp == null) ? 0 : emp.hashCode());
    return result;
}
@Override
public boolean equals(Object obj) {
    if (this == obj) {return true;}
    if (!(obj instanceof CohortGroup)) {return false;}
    CohortGroup other = (CohortGroup) obj;
    if(!getName().equals(other.getName())){return false;}
    if (emp == null && other.getOwner() != null) {
        return false;
    } else if (!emp.equals(other.getOwner())) {
        return false;
    }
    return true;
}

Employee

       @Override
public boolean equals(Object obj) {
    if (this == obj) {return true;}
    if (obj == null) {return false;}
    if (!(obj instanceof Employee)) {return false;}
    Employee other = (Employee) obj;
    if (EMPLID == null && other.getEMPLID() != null) {
        return false;
    } else if (!EMPLID.equals(other.getEMPLID())) {
        return false;
    }
    return true;
}

   @Override
public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + ((EMPLID == null) ? 0 : EMPLID.hashCode());
    return result;
}

I have added an addMember method to the CohortGroup that adds to both sides of the relationship:

    public void addMember(Employee emp){
    this.getMembers().add(emp);
    emp.getMemberGroups().add(this);

}

Continued thanks to all that are helping.

Answer 1

I highly suspect that you're not overriding equals and hashCode properly. Wrongly overriding them can lead to the kind of behavior you're experiencing (as the hash key is used as keys in maps). Double check what you did with equals and hashCode.

Using your annotated entities with good equals and hashCode, this code (logically equivalent):

Session session = HibernateUtil.beginTransaction();
Employee emp = (Employee) session.load(Employee.class, 1L);
CohortGroup group = (CohortGroup) session.load(CohortGroup.class, 1L);
group.getMembers().add(emp);
emp.getMemberGroup().add(group); // set the other side too!!
session.saveOrUpdate(group);
HibernateUtil.commitTransaction();

produces the following output on my machine:

08:10:32.426 [main] DEBUG o.h.e.d.AbstractFlushingEventListener - processing flush-time cascades
08:10:32.431 [main] DEBUG o.h.e.d.AbstractFlushingEventListener - dirty checking collections
08:10:32.432 [main] DEBUG org.hibernate.engine.CollectionEntry - Collection dirty: [com.stackoverflow.q2649145.CohortGroup.members#1]
08:10:32.432 [main] DEBUG org.hibernate.engine.CollectionEntry - Collection dirty: [com.stackoverflow.q2649145.Employee.memberGroup#1]
08:10:32.443 [main] DEBUG org.hibernate.engine.Collections - Collection found: [com.stackoverflow.q2649145.CohortGroup.members#1], was: [com.stackoverflow.q2649145.CohortGroup.members#1] (initialized)
08:10:32.448 [main] DEBUG org.hibernate.engine.Collections - Collection found: [com.stackoverflow.q2649145.Employee.memberGroup#1], was: [com.stackoverflow.q2649145.Employee.memberGroup#1] (uninitialized)
08:10:32.460 [main] DEBUG o.h.e.d.AbstractFlushingEventListener - Flushed: 0 insertions, 0 updates, 0 deletions to 2 objects
08:10:32.461 [main] DEBUG o.h.e.d.AbstractFlushingEventListener - Flushed: 0 (re)creations, 2 updates, 0 removals to 2 collections
08:10:32.463 [main] DEBUG org.hibernate.pretty.Printer - listing entities:
08:10:32.473 [main] DEBUG org.hibernate.pretty.Printer - com.stackoverflow.q2649145.CohortGroup{id=1, members=[com.stackoverflow.q2649145.Employee#1]}
08:10:32.474 [main] DEBUG org.hibernate.pretty.Printer - com.stackoverflow.q2649145.Employee{id=1, memberGroup=}
08:10:32.474 [main] DEBUG o.h.p.c.AbstractCollectionPersister - Inserting collection: [com.stackoverflow.q2649145.CohortGroup.members#1]
08:10:32.480 [main] DEBUG org.hibernate.jdbc.AbstractBatcher - about to open PreparedStatement (open PreparedStatements: 0, globally: 0)
08:10:32.491 [main] DEBUG org.hibernate.SQL - insert into MYSITE_RES_COHORT_GROUP_STAFF (COHORT_GROUPID, USERID) values (?, ?)
Hibernate: insert into MYSITE_RES_COHORT_GROUP_STAFF (COHORT_GROUPID, USERID) values (?, ?)
08:10:32.496 [main] TRACE org.hibernate.type.LongType - binding '1' to parameter: 1
08:10:32.497 [main] TRACE org.hibernate.type.LongType - binding '1' to parameter: 2
08:10:32.499 [main] DEBUG o.h.p.c.AbstractCollectionPersister - done inserting collection: 1 rows inserted

No delete before the insert!

By the way, note that you should set the link on both sides when working with bi-directional associations, like I did on the group and on the employee.

Or add defensive link management methods on your classes, for example on CohortGroup:

public void addToMembers(Employee emp) {
    this.getMembers().add(emp);
    emp.getMemberGroup().add(this);
}

public void removeFromMembers(Employee emp) {
    this.getMembers().remove(emp);
    emp.getMemberGroup().remove(this);
}