Is it possible to map a denormalized schema with duplicate rows to a single
enitity instance? (Background: our DB admins DO NOT want a mapping table for
m:n association. They prefer one denormalized table at one end of the
association from the performance perspective).
Let's assume this minimal example: The entites A and B have a bidirectional
m:n association and have to be persisted in these tables:
CREATE TABLE A {
A1 SMALLINT NOT NULL,
A2 VARCHAR(50),
PRIMARY_KEY(A1)
};
CREATE TABLE B {
B1 SMALLINT NOT NULL,
B2 SMALLINT NOT NULL,
A_ID SMALLINT NOT NULL,
PRIMARY_KEY(B1, A_ID)
};
The A side of the associtation is trivial:
@Entity
@Table(name = "A")
public class A {
@Id
private int a1;
private String a2;
@ManyToMany
private Collection bs;
How to mapped the B side?
@Entity
@Table(name = "B")
public class B {
@Id // ???
private int b1;
private int b2;
@ManyToMany
private Collection as;
>From the logical view, only b1 makes up the PK but technical it has to be
(b1, a_id).
When I insert/delete/update a B referencing more the one A, how will OpenJPA
know to persist m rows in table B? To make things clearer, lets assume the
following data set:
| A |
+----+----------+
| A1 | A2 |
+----+----------+
| 1 | foo |
| 2 | bar |
+----+----------+
| B |
+----+-----------+
| B1 | B2 | A_ID |
+----+-----------+
| 3 | 42 | 1 |
| 3 | 42 | 2 |
| 4 | 12 | 1 |
| 4 | 12 | 2 |
+----+-----------+
When I add a new B ...
A a1 = ....findById(1);
A a2 = ...findById(2);
B b = new B();
b.setB1(5);
b.setB2(33);
b.setAs(new ArrayList ());
b.getAs().add(a1);
b.getAs().add(a2);
em.persist(b);
... how do I define a mapping that INSERTs two rows
| 5 | 33 | 1 |
| 5 | 33 | 2 |
into table B? Of course, it's not only about the INSERT. All CRUD operations
and queries should honour the denormalization. Any ideas?
TIA,
Tobias
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