[hibernate-issues] [Hibernate-JIRA] Commented: (HHH-6911) Write DiscriminatorValue to DiscriminatorColumn when combined with InheritanceType#JOINED
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https://hibernate.onjira.com/browse/HHH-6911?page=com.atlassian.jira.plug...
]
Roberto Ruiz commented on HHH-6911:
-----------------------------------
There is situation when @DiscriminatorColumn in JOINED inheritance is quite useful. It
happens when the discriminator column is also a foreign key to another table (which may
contain the description of the subtype)
For example:
@Entity
@Table(name="CONTRACT")
@Inheritance(strategy=InheritanceType.JOINED)
@DiscriminatorColumn(name="ID_CONTRACT_TYPE",
discriminatorType=DiscriminatorType.INTEGER)
public class ContratoJoin {
@Id
@Column(name="ID_CONTRACT")
private Long id;
@Column(name="DESCRIPTION")
private String description;
@ManyToOne
@JoinColumn(name="ID_CONTRACT_TYPE", insertable=false, updatable=false)
private ContractType contractType;
//getters, setters
}
... subclasses with @DiscriminatorValue ...
@Entity
@Table(name="CONTRACT_TYPE")
public class ContractType {
@Id
@Column(name="ID_CONTRACT_TYPE")
private Long id;
@Column(name="DESCRIPTION")
private String description;
//getters, setters
}
In this case, the value of the column ID_CONTRACT_TYPE should be inserted automatically
depending on the subclass, so that the description is retrieved on query.
Currently, in order to do that I must set the ContractType manually. It's not much
work, nut it is error prone.
There is also an advantage when using discriminator column. I may look directly into the
CONTRACT table, and know the type.
Write DiscriminatorValue to DiscriminatorColumn when combined with
InheritanceType#JOINED
-----------------------------------------------------------------------------------------
Key: HHH-6911
URL: https://hibernate.onjira.com/browse/HHH-6911
Project: Hibernate ORM
Issue Type: Improvement
Components: annotations, metamodel
Reporter: Steve Ebersole
Priority: Minor
Labels: jpa2
The JPA spec states that {{DiscriminatorColumn}} can be used in conjunction with JOINED
inheritence. The only section that seems to deal with this combination is _11.1.10
DiscriminatorColumn Annotation_ where it says
{quote}
For the SINGLE_TABLE mapping strategy, and typically also for the JOINED strategy, the
persistence
provider will use a type discriminator column. The DiscriminatorColumn annotation is used
to
define the discriminator column for the SINGLE_TABLE and JOINED inheritance mapping
strategies.
{quote}
Apparently this is meant to imply that even if providers do not need/use the
discriminator for concrete class resolution (reading) they are still expected to write it
out to the database. Dunno, thats just what we have been told :)
Anyway, like I said, Hibernate itself does not need the discriminator column/value to
interpret the concrete classes. However, in the interest of portability we certainly
*could* write it out.
Longer term we could *possibly* even use the DiscriminatorColumn/DiscriminatorValue (if
present) instead of the SQL CASE statement thing we do now.
See HHH-4358 for the origins of this request...
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