java - Understanding mappedBy annotation in Hibernate -

i trying understand mappedby attribute of @onetomany annotation in jpa. created below example customer has 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 use hibernate generate tables see hibernate created 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 try save customer , orders see below dml statements generated hibernate:

hibernate: insert customer (id) values (?) hibernate: insert tbl_order (customer_id, ordernumber, id) values (?, ?, ?) hibernate: update tbl_order set orders_index=? id=? 

why hibernate tried insert , updates record in tbl_order instead of running single insert query?

now if remove mappedby attribute , try generate tables, see 3 tables 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 additional table in case? how mappedby attribute controlling this? why have additional unique key constraint on orders_id column in customer_tbl_order table?

now if try save customer , orders below dml operations:

hibernate: insert customer (id) values (?) hibernate: insert tbl_order (customer_id, ordernumber, id) values (?, ?, ?) hibernate: insert customer_tbl_order (customer_id, orders_index, orders_id) values (?, ?, ?) 

why don't have additional update query in case compared case have declared mappedby attribute?

that's normal.

with mappedby, directly tell hibernate/jpa 1 table owns relationship, , therefore stored column of table.

without, relationship external , hibernate/jpa need create table store relationship.

example:

• a stackoverflow question have several answer.
• an answer owned 1 , 1 question.

in plain jdbc, create 2 table:

questions(question_id, ...); answers(answer_id, question_id, ...); 

where question_id foreign key referencing question.question_id.

as other case, don't have real case since there every time many many a unique constraint (eg: question may have several answer, , answer may have physically have several question, appears once question).