Java Reference
In-Depth Information
private Collection<Item> itemCollection;
@JoinColumn(name = "CUSTOMER_ID",
referencedColumnName = "CUSTOMER_ID")
@ManyToOne
private Customer customerId;
//constructors, getters, setters, equals(), hashCode(), toString()
//omitted for brevity
}
Notice that the
CustomerOrder
entity has a property of type
java.util.Collection
named
itemCollection
. This property holds all items for the order. Notice that
the field is decorated with the
@ManyToMany
annotation, this annotation is used to
declare a many-to-many relationship between the
CustomerOrder
and
Item
JPA
entities. Notice that the field is also annotated with the
@JoinTable
annotation, this
annotation is necessary since a join table is necessary in a database schema whenever
there is a many-to-many relationship between tables. Using a join table allows us to
keep the data in the database normalized.
The
@JoinTable
annotation allows us to specify the table in the schema that is
used to denote the many-to-many relationship in the schema. The value of the
name
attribute of
@JoinTable
must match the name of the join table in the schema.
The value of the
joinColumns
attribute of
@JoinColumn
must be the foreign key
relationship between the join table and the owning side of the relationship. We
already discussed the
@JoinColumn
annotation when discussing one-to-many
relationships. In this case, its
name
attribute must match the name of the column in
the join table that has the foreign key relationship, and its
referencedColumnName
attribute must indicate the name of the primary key column on the owning side of
the relationship. The value of the
inverseJoinColumns
attribute of
@JoinTable
has a similar role as its
joinColumns
attribute, except it indicates the corresponding
columns for the non-owning side of the relationship.
The side of the many-to-many relationship containing the above annotations is
said to be the
owning side
of the relationship. Let's look at how the many-to-many
relationship is defined in the non-owning side of the relationship, which, in our case
is the
Item
JPA entity.
package com.ensode.jpa;
//imports omitted for brevity
@Entity
@Table(name = "ITEM")
@NamedQueries({
@NamedQuery(name = "Item.findAll",
query = "SELECT i FROM Item i"),
@NamedQuery(name = "Item.findByItemId",
