Java.Hibernate.Middle.What sql’s are executed when i select, insert, delete parent/child, especially when i need bidirectional connections ?

✅ Scenario: Mapping

You have:

@Entity
public class ParentEntity {
    @Id @GeneratedValue
    private Long id;

    @OneToMany(mappedBy = "parent", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<ChildEntity> children = new ArrayList<>();
}

@Entity
public class ChildEntity {
    @Id @GeneratedValue
    private Long id;

    @ManyToOne
    @JoinColumn(name = "parent_id")
    private ParentEntity parent;
}

📥 Insert Parent + Children

Code:

ParentEntity parent = new ParentEntity();
ChildEntity child1 = new ChildEntity();
ChildEntity child2 = new ChildEntity();

child1.setParent(parent);
child2.setParent(parent);
parent.getChildren().add(child1);
parent.getChildren().add(child2);

session.persist(parent);

SQL executed:
1️⃣ INSERT parent row:

insert into parent_entity (id, ...) values (1, ...);

2️⃣ INSERT each child row with foreign key:

insert into child_entity (id, parent_id, ...) values (10, 1, ...);
insert into child_entity (id, parent_id, ...) values (11, 1, ...);

📤 Select Parent + Children

Parent alone:

ParentEntity parent = session.find(ParentEntity.class, 1L);

SQL:

select ... from parent_entity where id=1;

✅ No child data loaded if collection is LAZY → Hibernate will proxy the children.

Accessing children:

parent.getChildren().size(); // triggers child collection loading

SQL:

select ... from child_entity where parent_id=1;

🗑️ Delete Parent

session.remove(parent);

If you have cascade = CascadeType.ALL + orphanRemoval = true, Hibernate issues:

1️⃣ Delete all child rows first:

delete from child_entity where parent_id=1;

2️⃣ Delete parent row:

delete from parent_entity where id=1;

✅ This ensures referential integrity → avoids foreign key constraint violations.

🔄 Remove Child from Parent Collection

parent.getChildren().remove(child1);

If orphanRemoval = true, Hibernate issues:

delete from child_entity where id=10;

Otherwise (orphanRemoval = false), Hibernate updates the child to detach it:

update child_entity set parent_id=null where id=10;

📝 Update Child

child1.setSomeField("newValue");

Hibernate issues:

update child_entity set some_field='newValue' where id=10;

🕵️ Special Note on Bidirectional Consistency

Hibernate does not issue extra SQL to “sync” Java-side parent-child references.
It’s your responsibility to maintain consistency:

child.setParent(parent);
parent.getChildren().add(child);

📌 Key Takeaways

✅ Insert parent + children → INSERT parent → INSERT children with parent_id.
✅ Select parent → one SELECT on parent; child collection triggers separate SELECT if accessed (unless using fetch join).
✅ Delete parent → DELETE children (if cascade+orphanRemoval) → DELETE parent.
✅ Remove child from collection → DELETE child or set parent_id=null depending on orphanRemoval.
✅ Bidirectional mappings require you to sync both sides in Java objects — Hibernate does not auto-sync Java references.