I am trying to understand the mappedBy attribute of @OneToMany annotation in JPA. I created below example where a Customer has a list of Orders:
@Entity
public class Customer {
@Id @GeneratedValue public Integer getId() { return id; }
public void setId(Integer id) { this.id = id; }
private Integer id;
@OneToMany(mappedBy="customer")
@OrderColumn(name="orders_index")
public List<Order> getOrders() { return orders; }
public void setOrders(List<Order> orders) { this.orders = orders; }
private List<Order> orders;
}
@Entity
@Table(name="TBL_ORDER")
public class Order {
@Id @GeneratedValue public Integer getId() { return id; }
public void setId(Integer id) { this.id = id; }
private Integer id;
public int getOrderNumber() { return orderNumber; }
public void setOrderNumber(int orderNumber) { this.orderNumber = orderNumber; }
private int orderNumber;
@ManyToOne
public Customer getCustomer() { return customer; }
public void setCustomer(Customer customer) { this.customer = customer; }
private Customer customer;
}
Now when I use Hibernate to generate the tables then I see that Hibernate created only 2 tables:
Hibernate: create table Customer (id number(10,0) not null, primary key (id))
Hibernate: create table TBL_ORDER (id number(10,0) not null, orderNumber number(10,0) not null, customer_id number(10,0), orders_index number(10,0), primary key (id))
Hibernate: alter table TBL_ORDER add constraint FK_nt24krtgqwcsynosqgk4jkvfv foreign key (customer_id) references Customer
Also if I try to save a customer and some Orders I see below DML statements generated by Hibernate:
Hibernate: insert into Customer (id) values (?)
Hibernate: insert into TBL_ORDER (customer_id, orderNumber, id) values (?, ?, ?)
Hibernate: update TBL_ORDER set orders_index=? where id=?
Why hibernate tried to insert and updates the record in TBL_ORDER instead of just running a single insert query?
Now if I remove the mappedBy attribute and try to generate the tables, I see 3 tables this time:
Hibernate: create table Customer (id number(10,0) not null, primary key (id))
Hibernate: create table Customer_TBL_ORDER (Customer_id number(10,0) not null, orders_id number(10,0) not null, orders_index number(10,0) not null, primary key (Customer_id, orders_index))
Hibernate: create table TBL_ORDER (id number(10,0) not null, orderNumber number(10,0) not null, customer_id number(10,0), primary key (id))
Hibernate: alter table Customer_TBL_ORDER add constraint UK_sw94jktvh72tripj876s31052 unique (orders_id)
Hibernate: alter table Customer_TBL_ORDER add constraint FK_sw94jktvh72tripj876s31052 foreign key (orders_id) references TBL_ORDER
Hibernate: alter table Customer_TBL_ORDER add constraint FK_f03up2h945cg0dcbo2pdb1d3c foreign key (Customer_id) references Customer
Hibernate: alter table TBL_ORDER add constraint FK_nt24krtgqwcsynosqgk4jkvfv foreign key (customer_id) references Customer
Why hibernate creates an additional table in this case? How the mappedBy attribute is controlling this? Why we have an additional unique key constraint on orders_id column in Customer_TBL_ORDER table?
Now if I try to save a customer and its orders I get below DML operations:
Hibernate: insert into Customer (id) values (?)
Hibernate: insert into TBL_ORDER (customer_id, orderNumber, id) values (?, ?, ?)
Hibernate: insert into Customer_TBL_ORDER (Customer_id, orders_index, orders_id) values (?, ?, ?)
Why we don't have an additional update query in this case compared to the case where I have declared the mappedBy attribute?
