Non-owning entity side of @OneToOne/@ManyToOne/@ManyToMany

Question

I am trying to understand the javax.persistence annotations @OneToOne , @ManyToOne and @ManyToMany . The descriptions for these annotations make a mention of non-owning side . Specifically:

[@OneToOne]: If the relationship is bidirectional, the non-owning side must use the mappedBy element of the OneToOne annotation to specify the relationship field or property of the owning side.

[@ManyToOne]: If the relationship is bidirectional, the non-owning OneToMany entity side must used the mappedBy element to specify the relationship field or property of the entity that is the owner of the relationship.

[@ManyToMany]: If the relationship is bidirectional, the non-owning side must use the mappedBy element of the ManyToMany annotation to specify the relationship field or property of the owning side.

I am having trouble understanding this ownership aspect. For example, I have the following associations:

Note: Images taken from here.

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So which are the non-owning entity sides of these associations?

Answer 1

In the bi-directional relationship betweens two objects ,you have to choose which sides to manage the relationship. From the database perspective , managing the relationship means managing the value of some FK column that link between two tables. The side that managing it is called owning side. Otherwise, it is call non-owning side.

So back to your example on ProjectManager and Project . Which object is the owning side depends on which object you choose to manage their relationship.

If you choose ProjectManager to be the owning side (hence Project is the non-owning side), only the values of ProjectManager#getProjects() will be used to determine the value of such FK column. (ie project table 's project_manager_id column in this case) The value of Project#getProjectManager() will be ignored and does not affect the value of this FK column.

In term of JPA mapping , it is :

@Entity
@Table(name="project_manager")
public class ProjectManager{

    @OneToMany
    private List<Project> projects = new ArrayList<>();

}

@Entity
@Table(name="project")
public class Project {

    @ManyToOne
    @JoinColumn(name = "project_manager_id")
    private ProjectManager projectManager;
}

On the other hands , if you choose Project to the owning side (hence ProjectManager is the non-owning side) , only the value of Project#getProjectManager() will be used to determine the value of this FK column while the value of ProjectManager#getProjects() will be ignored. The JPA mapping in the case will be :

@Entity
@Table(name="project_manager")
public class ProjectManager{

    @OneToMany(mappedBy="projectManager")
    private List<Project> projects = new ArrayList<>();

}

@Entity
@Table(name="project")
public class Project {

    @ManyToOne
    @JoinColumn(name = "project_manager_id")
    private ProjectManager projectManager;
}

_{PS: I explain it using property access , hopefully you should get the idea.}