I'm using single table inheritance, one parent entity:
@Entity
@Table(name = "parent")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "type")
public class Parent {
// ...
}
and two child entities:
@Entity
@DiscriminatorValue("child1")
public class Child1 extends Parent {
@Column(name = "child1_property")
private Integer child1Property;
// ...
}
@Entity
@DiscriminatorValue("child2")
public class Child2 extends Parent {
@Column(name = "child2_property")
private Integer child2Property;
// ...
}
Now, if I query (HQL) directly from Child1
entity:
from Child1
it will generate an SQL that selects only the columns from Parent
entity plus Child1
entity. If I select from Parent
entity using a where type='child1'
from Parent p where type(p)='child1'
it will return only Child1
entities but the SQL query selects all columns from Parent
, Child1
and Child2
. Is there a possibility to query from Parent
entity use the discriminator column (i.e. limit only to Child1
entities) and get a specific SQL query, i.e. that will select only columns from Parent
and Child1
.
A projection seems to work:
SELECT new fully.qualified.name.of.Child1(p.id, p.name, p.child1Property)
FROM Parent p
WHERE TYPE(p) = 'child1'
produces
select
parent0_.id as col_0_0_,
parent0_.name as col_1_0_,
parent0_.child1_property as col_2_0_
from
parent parent0_
where
parent0_.type='child1'
Hibernate User Guide excerpt:
There is a particular expression type that is only valid in the select clause. Hibernate calls this "dynamic instantiation". JPQL supports some of that feature and calls it a "constructor expression".
So rather than dealing with the Object[] (again, see Hibernate Query API) here, we are wrapping the values in a type-safe Java object that will be returned as the results of the query.
...
The projection class must be fully qualified in the entity query, and it must define a matching constructor.
The class here need not be mapped. It can be a DTO class.
If it does represent an entity, the resulting instances are returned in the NEW state (not managed!).
However, I'd just SELECT c FROM Child1 c
. I was actually surprised that Hibernate didn't complain about the attribute child1Property
missing for the entity Parent
.