Java Reference
In-Depth Information
Notice that the field is decorated with a
@ManyToOne
annotation. This annotation
marks the
many
side of the one-to-many relationship between
Customer
and
Address
. Notice that the field is also decorated with the
@JoinColumn
annotation.
The
name
attribute of this annotation indicates the column in the database our
entity maps to that defines the foreign key constraint between the
ADDRESS
and
CUSTOMER
tables (in our case, the
CUSTOMER_ID
column on the
ADDRESS
table). The
referencedColumnName
attribute of
@JoinColumn
is used to indicate the primary
key column of the table on
one
side of the relationship (in our case, the
CUSTOMER_ID
column in the
CUSTOMER
table).
In addition to one-to-many and many-to-one relationships, JPA provides annotations
to denote many-to-many and one-to-one relationships. In our schema, we have a
many-to-many relationship between the
CUSTOMER_ORDER
and
ITEM
tables; since
an order can have many items, and an item can belong to many orders.
The table to hold orders was named
CUSTOMER_ORDER
since the word
"order" is a reserved word in SQL.
Let's take a look at the
CustomerOrder
JPA entity to see how the many-to-many
relationship is defined:
package com.ensode.jpa;
import java.io.Serializable;
import java.util.Collection;
import javax.persistence.Basic;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.ManyToOne;
import javax.persistence.NamedQueries;
import javax.persistence.NamedQuery;
import javax.persistence.Table;
import javax.validation.constraints.NotNull;
import javax.validation.constraints.Size;
@Entity
@Table(name = "CUSTOMER_ORDER")
@NamedQueries({
