Collection mappings - Collections

• Collection mappings include OneToMany, ManyToMany,
• JPA requires that the type of the collection field or get/set methods be one of the Java collection interfaces, Collection, List, Set, or Map.

Collection Implementations

• Your field should not be of a collection implementation type, such as ArrayList.

Duplicates

• A List in Java supports duplicate entries, and a Set does not.
• In the database, duplicates are generally not supported.
•  Technically it could be possible if a JoinTable is used, but JPA does not require duplicates to be supported, and most providers do not.
• If you require duplicate support, you may need to create an object that represents and maps to the join table. This object would still require a unique Id, such as a GeneratedValue. 

Ordering

• JPA allows the collection values to be ordered by the database when retrieved.
• This is done through the @OrderBy annotation or <order-by> XML element.
• The value of the OrderBy is a JPQL ORDER BY string. 
• Attribute name followed by ASC or DESC for ascending or descending ordering. 
• If no OrderByvalue is given it is assumed to be the Id of the target object.
• The OrderBy value must be a mapped attribute of the target object.
• Using an OrderBy does not ensure the collection is ordered in memory.You are responsible for adding to the collection in the correct order. 
• Java does define a SortedSet interface and TreeSet collection implementation that does maintain an order.
• You can also use the Collections.sort() method to sort a List when required. 

Example of a collection order by annotation

@Entity
public class Employee {
  @Id
  private long id;
  ...
  @OneToMany
  @OrderBy("areaCode")
  private List<Phone> phones;
  ...
}

The OrderColumn is maintained by the mapping and should not be an attribute of the target object. The table for the OrderColumn depends on the mapping. For a OneToMany mapping it will be in the target object's table. For aManyToMany mapping or a OneToMany using a JoinTable it will be in the join table. For an ElementCollection mapping it will be in the target table.

Example of a collection order column database
EMPLOYEE (table)

ID
FIRSTNAME
LASTNAME
SALARY
1
Bob
Way
50000
2
Sarah
Smith
60000
EMPLOYEE_PHONE (table)

EMPLOYEE_ID
PHONE_ID
INDEX
1
1
0
1
3
1
2
2
0
2
4
1
PHONE(table)

ID
AREACODE
NUMBER
1
613
792-7777
2
416
798-6666
3
613
792-9999
4
416
798-5555

Example of a collection order column annotation
@Entity
public class Employee {
  @Id
  private long id;
  ...
  @OneToMany
  @OrderColumn(name="INDEX")
  private List<Phone> phones;
  ...
}