Selecting MappedSuperclass from the database (Hibernate)

Question

Problem

I have a @MappedSuperclass called Data as the parent of every Entity in my database. It contains common attributes like Id etc. I then have an

Answer 1

I seem to be able to do this (although using InheritanceType.JOINED) with hibernate 5.0.8, java 1.8.0_73 and Oracle 12c - either I am misunderstanding or maybe hibernate has changed..

I have the following hierarhcy:

@MappedSuperclass
@Inheritance(strategy=InheritanceType.JOINED)
CommonRoot
 |
 |  @MappedSuperclass
 +- Mapped
        |  @Entity(name="Concrete1")
        |  @Table(name="CON1")
        +- Concrete1
        |
        |  @Entity(name="Concrete2")
        |  @Table(name="CON2")
        +- Concrete2

And I can do the following HQL:

SELECT entityId FROM com.hibernatetest.Mapped ORDER BY entityId ASC

which gives these 2 SQL statements:

select concrete2x0_.entityId as col_0_0_ from CON2 concrete2x0_ order by concrete2x0_.entityId ASC
select concrete1x0_.entityId as col_0_0_ from CON1 concrete1x0_ order by concrete1x0_.entityId ASC

and the warning

WARN: HHH000180: FirstResult/maxResults specified on polymorphic query; applying in memory!

Not sure what they mean though, as this can be done with SQL as:

(select entityId from CON2
union all
select entityId from CON1)
order by entityId ASC

(And you can also add limit/rownum clauses to that if you desire, although that gets a bit clunky:

select * from (
(select * from (select entityId from CON2 order by entityId ASC) where rownum <= 10)
UNION ALL
(select * from (select entityId from CON1 order by entityId ASC) where rownum <= 10)
) where rownum <= 10 order by entityId ASC

Not sure why hibernate shouldn't be able to do this - might suggest it to them.)

Answer 2

No if you are using @MappedSuperclass

The reason for this is that when you define base class as @MappedSuperclass, there is no table generated for the base class, instead all of the properties are replicated in the concrete tables. In your example only FullTimeEmployee, PartTimeEmployee and Store tables would exist.

If you want to be able to query for base class entities you need to select different mapping for base classes. Use @Inheritance annotation on the base class and select one of the 3 possible mapping strategies - SINGLE TABLE, TABLE PER CLASS or JOINED

Answer 3

Yes

FROM Employee WHERE Employee.<employee only properties> = someValue

But only, as the others have said here, if the Employee entity is mapped. You don't need to even map it to its' own table. See the mapping strategies in Hibernate.