Cascading a Child entity persist operation to its Parent entity

问题

I have a OneToMany association with two entities that I have set up using Hibernate annotations. The Child entity of this association has a composite primary key that consists of the foreign key parent column and another identifier childName. This seems to cause a "Referential integrity constraint violation" when I attempt to cascade a commit to the parent by saving the child entity.

I have created a simple working example of the issue that abstracts away from the actual scenario that I am modelling with this relationship, but it means that I know the problem is due to this association and using a composite primary key.

Why can I not commit both entities by saving the child entity? Why does it violate foreign key constraints?

Main test method*:

// Create some detached entities
Parent p = new Parent();
p.setName("Fooson");

// New child id
Child.ChildPK pk = new Child.ChildPK();
pk.setParentName(p);
pk.setChildName("Barty");     

// Set id to new Child
Child c = new Child();
c.setChildPK(pk);

// Add child to parent
p.getChildren().add(c);

// Saving the parent
// service.parentDao.save(p); // if this is uncommented, it works fine

// Cascade child and associated parents in one transaction
service.childDao.save(c); // ConstraintViolationException

Child.java

@Entity
@Table(name="Child")
public class Child implements Serializable {

    @EmbeddedId
    private ChildPK id;

    public ChildPK getChildPK() {
        return id;
    }

    public void setChildPK(ChildPK childPK) {
        this.id = childPK;
    }

    @Embeddable
    public static class ChildPK implements Serializable 
    {
        @ManyToOne(cascade=CascadeType.ALL)
        @JoinColumn(name="name")
        Parent parent;

        @Column(name="childName")
        String childName;

        public Parent getParentName() {
            return parent;
        }

        public void setParentName(Parent parentName) {
            this.parent = parentName;
        } 

        public String getChildName() {
            return childName;
        }

        public void setChildName(String childName) {
            this.childName = childName;
        }

        @Override
        public int hashCode() {
            int hash = 5;
            hash = 67 * hash + Objects.hashCode(this.parent);
            hash = 67 * hash + Objects.hashCode(this.childName);
            return hash;
        }

        @Override
        public boolean equals(Object obj) {
            if (obj == null) {
                return false;
            }
            if (getClass() != obj.getClass()) {
                return false;
            }
            final ChildPK other = (ChildPK) obj;
            if (!Objects.equals(this.parent, other.parent)) {
                return false;
            }
            return Objects.equals(this.childName, other.childName);
        }

        @Override
        public String toString() {
            return "ChildPK{" + "parentName=" + parent.getName() + ", childName=" + childName + '}';
        }

    }

    @Override
    public String toString() {
        return "Child{" + "id=" + id + '}';
    }

}

Parent.java

@Entity
@Table(name="Parent")
public class Parent implements Serializable {

    @Id
    @Column(name="name")
    private String name;

    @OneToMany(mappedBy="id.parentName", cascade=CascadeType.ALL)
    Set<Child> children = new HashSet<>(0);

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Set<Child> getChildren() {
        return children;
    }

    public void setChildren(Set<Child> children) {
        this.children = children;
    }

    @Override
    public int hashCode() {
        int hash = 3;
        hash = 73 * hash + Objects.hashCode(this.name);
        return hash;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj == null) {
            return false;
        }
        if (getClass() != obj.getClass()) {
            return false;
        }
        final Parent other = (Parent) obj;
        if (!Objects.equals(this.name, other.name)) {
            return false;
        }
        return true;
    }

    @Override
    public String toString() {
        return "Parent{" + "name=" + name + ", children=" + children + '}';
    }

}

The complete exception message is:

Exception in thread "main" org.hibernate.exception.ConstraintViolationException: 
Could not execute JDBC batch update
...
Caused by: org.h2.jdbc.JdbcBatchUpdateException: 
Referential integrity constraint violation: 
"FK3E104FCBA07683D: PUBLIC.CHILD FOREIGN KEY(NAME) REFERENCES 
PUBLIC.PARENT(NAME) ('Fooson')"; SQL statement:
insert into Child (childName, name) values (?, ?)
...

---

* the main method refers to DAO and ServiceLayer objects that I have not shown here because I don't think it is relevant to the problem. I can, however, post these if asked for.

回答1:

It doesn't make sense to cascade a save operation from a Child to a Parent entity, it's the other way around.

That's because the operations will be:

• save the Child
• propagate the save operation to Parent

but the Child depends on the Parent.id for setting its FK.

You need to visualize this operation flow from the database perspective too. Can you save a Child who references a non-existing Parent? Of course, not.

Think what will happen when you delete a Child entity. The cascade will propagate to the Parent and then to all its Children.

So, you have to remove the cascading from the Embeddable class:

@ManyToOne(cascade=CascadeType.ALL)

and uncomment the code that works fine:

// Saving the parent
service.parentDao.save(p); // if this is uncommented, it works fine

来源：https://stackoverflow.com/questions/28519025/cascading-a-child-entity-persist-operation-to-its-parent-entity