hibernate 1:M relationships & cascade

[Sol Mayer-Orn]

Hi,
I've run into a problem with hibernate relationships.
It only appears to occur when using a 1-to-many, bi-directional relationship with *cascade*.
Tried posting this question to the hibernate forum, but was bitterly disappointed by their attitude (admittedly they have a right to it, as they're not being payed).
Would any of you veteran hibernate users happen to find the time to try it out ? If so, i'd be forever in your debt...
-----------------------------------------
Problem in short: when using 1:M, bi-directional relationship with "cascade", hibernate saves the parent but fails to save children.
I shall demonstrate for relationship "Department has several Employees".
-----------------------------------------
The classes:
------------------------

  
public class Employee { 
   String id; 
   double salary; 
   Department  dept; 
   // Constructors 
   // Setters, getters , equals, hashCode 
} 
public class Department { 
   String id; 
   Set employees; 
   // Constructors 
   // setters, getters, equals, hashCode 
    
   public void addEmployee(Employee e){ 
       employees.add(e); 
       e.setDept(this);     // bi-directional 
   } 
} 

-----------------------------------------
The mapping (bi-di relashionship, with "inverse=true" and "cascade="all":
------------------------

<?xml version="1.0"?> 
<!DOCTYPE hibernate-mapping SYSTEM "hibernate-mapping-2.0.dtd"> 
<hibernate-mapping package="test"> 
    <class name="Employee" table="Employee"> 
        <id name="id" column="id" type="string" > 
            <generator class="assigned"/> 
        </id> 
       <property name="salary" /> 
       <many-to-one name="dept" class="Department" column="dept_id"  /> 
   </class>    
   <class name="Department" table="Dept"> 
       <id name="id" column="id" type="string" > 
            <generator class="assigned"/> 
       </id> 
       <set name="employees" inverse="true" cascade="all"> 
             <key column="dept_id"/> 
             <one-to-many class="Employee" /> 
       </set>       
    </class> 
</hibernate-mapping> 

------------------------
Oracle tables:
------------------------

create table Dept(
id varchar(50) primary key,
descr varchar(50)
);
create table Employee(
id varchar(50) primary key,
salary number,
dept_id varchar(50) , 
constraint Emp_dept_id foreign key (dept_id) references dept(id)
);

------------------------
Usage:
------------------------

public static void main(String[] args) throws Exception{ 
    Connection con = DriverManager.getConnection(url, user, pass); 
    Configuration hcfg=new Configuration(); 
    hcfg=hcfg.configure(new File("hcfg.xml")); 
    SessionFactory fact= hcfg.buildSessionFactory();       
    Session session = fact.openSession(con); 
    Department dept = new Department("d1"); 
    Employee  emp=new Employee("e1", 9000);       
    Employee  emp2=new Employee("e2", 8900); 
    dept.addEmployee(emp); 
    dept.addEmployee(emp2);       
    session.save(dept); 
    session.save(emp);  // Redundant (it should be enough to save "dept")
    session.save(emp2); // However, employees are not saved with this
                        // explicit save 
    session.flush(); 
} 

------------------------
The problem:
------------------------
Department is saved to the DB, but child employees are not. According to the log, hibernate issues an sql "update" instead of "insert".
Interesting notes:
1) Problem only occurs with "cascade". When removing the cascade, you need to explicitly save each employee, but at least it works.
2) Behaviour differs if you save the new department before adding employees, namely:
session.save(dept); dept.addEmployee(emp); session.save(emp);
Again, any help would be greatly appreciated.

[Lasse Koskela]

Hi Sol,
I noticed your display name features an initial for a last name, which goes against our naming policy. Would you mind expanding it into a full last name? You can do that via My Profile.
Thanks.

[Paul Sturrock]

If you are seeing updates instead of inserts it sounds like there might be a problem with the id of the object (i.e. Hibernate has a reference to an existing object, rather than a new one). You should be able to see what values Hibernate thinks its got with debug turned on. Check these. I suggest this because I notice you are overriding equals and hashCode - something I'm not strictly sure you need to do - what's your reasoning here?

[Sol Mayer-Orn]

Thanx for having a look.
1)The equals/hash are not necessary, I get the same behaviour when removing them.
2)Unsurprisingly, the log says "saveOrUpdate() previously saved instance with id: e1" , which is why it chooses to run an "update" rather than "insert" on those employees.
3)The fact those employees are marked as "already saved" appears to be somethow related to the fact it's been added to department (even though department itself was not saved yet). If you don't add them to department they get saved fine:
dept = new Department("d1");
emp=new Employee("e1", 9000);
// dept.addEmployee(emp); // If you rem this out, emp gets saved.
session.save(dept);
session.save(emp);
Thanx again.

[Sol Mayer-Orn]

Thanx - your tip on the log & id's enabled me to send more focused info to the hibernate forum. Their reply indicates that the problem is with the "assigned" id. If I understand correctly (i could be wrong), they seem to assume that if an object has an id (different from 'null' or from the 'unsaved-value'), then it's an existing DB record. Which means if you manually assign the id before inserting, then hibernate would get confused. They suggested to add a "version" mapping, and it helped.
Still, seems like an odd feature to me. I would have expected hibernate to rely on some flag, separate from the id value...

Thanx again

[karl koch]

hi

is there a reason you want to assign the ids by yourself ?
if no, just let hibernate take care of them:

 
        <id
            name="id"
            column="ID"
            type="java.lang.Long"
            unsaved-value="null"
        >
            <generator class="hilo">
            </generator>
        </id>
 
 

(there are also generators for string ids)


karl

[Sol Mayer-Orn]

Thanx for relying.
Assinged IDs are required since we're using a legacy system which relies on natural keys (e.g. people stored by social security number, equipment items stored by manufacturer-issued id's, etc).

[Brendan Richards]

I've found an FAQ entry that has some good info on this topic:

http://www.hibernate.org/116.html#A12

[Balakishnan Ramaswamy]

Hi Sol Mam-Orn,


I'm facing the same problem. Did you manage to solve it? If so, please throw some light.

Many thanks in advance

Regards,
Bala