BiDirectional OneToMany Associations and Ownership

tzman · **Joined:** Fri Jan 16, 2004 11:05 am **Posts:** 33

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Hibernate version:
Hibernate 3.2cr2
Hibernate Annotations 3.2 cr1

We have some confusion here as to what is meant by owner side of a relationship in the hibernate_annotations.pdf reference on page 18.

Both examples are using the same Objects making it unclear, atleast to us.

From the reference manual page 18.
"Since many to one are (almost) always the owner side of a bidirectional relationship in the EJB3 spec, the one
to many association is annotated by @OneToMany( mappedBy=... )"

Code:

@Entity
public class Troop {
@OneToMany(mappedBy="troop")
public Set<Soldier> getSoldiers() {
...
}
@Entity
public class Soldier {
@ManyToOne
@JoinColumn(name="troop_fk")
public Troop getTroop() {
...
}

"Troop has a bidirectional one to many relationship with Soldier through the troop property. You don't have to
(must not) define any physical mapping in the mappedBy side."

Is this refering to the database ownership or the logical ownership?

From the reference manual page 19.
"To map a bidirectional one to many, with the one-to-many side as the owning side, you have to remove the
mappedBy element and set the many to one @JoinColumn as insertable and updatable to false. This solution is
obviously not optimized and will produce some additional UPDATE statements."

Code:

@Entity
public class Troop {
@OneToMany
@JoinColumn(name="troop_fk") //we need to duplicate the physical information
public Set<Soldier> getSoldiers() {
...
}
@Entity
public class Soldier {
@ManyToOne
@JoinColumn(name="troop_fk", insertable=false, updatable=false)
public Troop getTroop() {
...
}

Is the reference to "one-to-many" meaning the side annotated with @OneToMany?

We are trying to determine which way the mapping should be done if logically we say that a Troop owns a set of Sodiers. Which also means that to create a new Soldier you would have to do this by adding the Soldier to the Troop. So, the child is always added via the parent, it can never be added on its own.

However, from a database point of view, the Soldier is the owner because he is the one holding the foreign key.

Hopefully this question makes sense and I appreciate the clarification

tzman · **Joined:** Fri Jan 16, 2004 11:05 am **Posts:** 33

To make this more clear I think the difference is in the logical relationship.

Given the following UML:

Would be the first example in the manual and mapped as following:

Code:

@Entity 
public class Troop { 
@OneToMany(mappedBy="troop") 
public Set<Soldier> getSoldiers() { 
... 
} 
@Entity 
public class Soldier { 
@ManyToOne 
@JoinColumn(name="troop_fk") 
public Troop getTroop() { 
... 
} 

Given the following UML:

Would be the second example in the manual and mapped as following:

Code:

@Entity 
public class Parent{ 
@OneToMany 
@JoinColumn(name="parent_fk") //we need to duplicate the physical information 
public Set<Offspring> getOffspring() { 
... 
} 
@Entity 
public class Offspring{ 
@ManyToOne 
@JoinColumn(name="parent_fk", insertable=false, updatable=false) 
public Parent getParent() { 
... 
} 

Is this correct?

Hopefully this made the situation more clear.

emmanuel · **Posted:** Fri Jul 28, 2006 2:14 pm

Composition and aggregation have nothing to do with assoication ownership.
association ownership means when I update the owner side in my object model, the change will trigger a database update of the FK(s).

There is no logical concept in it