JPA CriteriaBuilder left outer join for table with no relations?

Question

Initially I had requirement to write code using JPA CriteraiBuilder for following SQL:

SELECT ve.col_1,
  (SELECT vm.col_4
  FROM table2 vm
  WHERE vm.col_2  = ve.col_2
  AND vm.col_3 = ve.col_3
  ) as col_a
FROM table1 ve;

But I learnt that, it is not possible to add subquery in select clause. So I changed my query to use left outer join like this.

SELECT ve.col_1,
  vm.col_4 as col_a
FROM table1 ve,
  table2 vm
WHERE 
vm.col_2 (+)    = ve.col_2 
AND vm.col_3 (+) = ve.col_3;

Now table1 and table2 do not have direct relations using foreign keys. Corresponding JPA entities look like:

Table1.java ->

@Column(name = "COL_1")
private String col_1;

@Column(name = "COL_2")
private String col_2;

@Column(name = "COL_3")
private String col_3;

@OneToOne(fetch = FetchType.LAZY)
@JoinColumns({
    @JoinColumn(name="COL_2"),
    @JoinColumn(name="COL_3")
})
private Table2 table2;


Table2.java ->

@Column(name = "COL_4")
private String col_4;

@Column(name = "COL_2")
private String col_2;

@Column(name = "COL_3")
private String col_3;

My code looks like:

final CriteriaBuilder criteriaBuilder = entityManager.getCriteriaBuilder();

final CriteriaQuery<SearchTO> query = criteriaBuilder.createQuery(
        SearchTO.class);
Root<Table1> root = query.from(Table1.class);

final Join<Table1, Table2> joinTable2 = root.join(Table1_.table2,
        JoinType.LEFT);

Then I am trying to fetch value using:

joinTable2.get(Table2_.col_4)

Then now I am getting error as:

A Foreign key refering com.Table2 from com.Table1 has the wrong number of column

Table2 has around 6 columns with annotation @Id and I can not change change it to have only two columns with @Id annotation.

Please let me know:

• If it is possible to write code using CriteriaBuilder for my approach 1 (subquery in select clause).

• If thats not possible, how can I implement this left outer join as mentioned for approach 2. Please note that Table2 does not have any references of Table1.

Please note that I am using plain JPA APIs. DB is Oracle11g. JDK version is 1.7.

Answer 1

For Approach 1: You can write subquery for criteria query

Subquery<Entity1> subquery = cq.subquery( Entity1.class );
    Root fromSubQuery = subquery.from( Entity1.class );
    subquery.select( cb.max( fromSubQuery.get( "startDate" ) ) );
    subquery.where( cb.equal( fromSubQuery.get( "xyzId" ), fromRootOfParentQuery.get( "xyzId" ) ) );

Use it as :

Root<Entity2> entity2 = cq.from( Entity2.class );
Predicate maxDatePredicate = cb.and( cb.equal( entyty2.get( "startDate" ), subquery ) );

For Approach 2: There is no other way than having relationship between two entities for left join. You can define private variable for relationship without getter & setter and use that variable for setting left join. Then add the predicate to criteriaBuilder