CriteriaBuilder query with sub-query, single column result, max function and generic types using metamodel - criteria

Question

My objective is to replace an old JPQL query with a generic type-safe helper method using javax.persistence.metamodel and javax.persitence.criteria.
The query is essentially
select * from table
where field1 = arg1
and field2 = arg2
and field3 = (select max (field3)
from table
where field1 = arg1
and field2 = arg2
and field3 <= arg3
)
Admittedly this is maybe too specialized a query to generalize but I see the need for 2 or 3 other more generic helpers which I can model on this solution.
I have been googling the Criteria documentation (one problem is it's easy to surf a google search result list and mistakenly move from a javax.persitence page to a JBoss Hibernate page... and they are NOT the same).
I have obviously not found a one-stop shop that tells me all I need to know:
how to select a single field in a CriteriaQuery
how to structure a subquery in a CriteriaQuery Expression
how to write a max aggregate function call using CriteriaBuilder
how to properly use Static Metamodel attributes to specify generic classes in a CriteriaBuilder query, when the table being queried has a composite key which is mapped by composite key class (using #EmbeddedId)

Answer 1

OK. I already had the answer before I posted the question but I thought it might be useful to publish what I found.
The use case is a CHARGE table that provides CHG_NU values for ranges of product-option-level values. The appropriate
CHG_NU from the table is the one that matches a PROD_CD and OPTION_TYPE and does not exceed the OPTION_LEVEL.
Here's the method I ended up writing (the comments are specific to the above use-case but the code is generic):
public static <X, KT, PT, BT, NT extends Number> X findWithUpperLimit (Class<X> rootClass, Class<NT> numericClass,
SingularAttribute<X, KT> keyAttr,
SingularAttribute<KT, PT> arg1Attr, PT arg1Val,
SingularAttribute<KT, BT> arg2Attr, BT arg2Val,
SingularAttribute<KT, NT> numericAttr, NT number,
EntityManager em)
{
List<X> results;
CriteriaBuilder cb = em.getCriteriaBuilder ();
// set up the query (returns a full record of the CHARGE table)...
CriteriaQuery<X> cq = cb.createQuery (rootClass);
// ... and the subquery (returns only the BigDecimal OPT_LEVEL)
Subquery<NT> sq = cq.subquery (numericClass);
// set up the root objects for the CHARGE table. Both the query and the subquery are on the same table
Root<X> root = cq.from (rootClass);
Root<X> sqRoot = sq.from (rootClass);
// the query objects and the criteria builder are used to structure the query,
// the root objects are used to get metadata from the table to assign table elements to the criteria
// the subquery gets the closest optLevel to the passed-in number...
sq.select (cb.max (sqRoot.get (keyAttr).get (numericAttr)))
.where (cb.and
(cb.equal (sqRoot.get (keyAttr).get (arg1Attr), arg1Val),
cb.equal (sqRoot.get (keyAttr).get (arg2Attr), arg2Val),
cb.le (sqRoot.get (keyAttr).get (numericAttr), number)
));
// ...and the main query matches the passed-in prodCd, optType and the optLevel found by the subquery.
cq.select (root).where (cb.and (cb.equal (root.get (keyAttr).get (arg1Attr), arg1Val),
cb.equal (root.get (keyAttr).get (arg2Attr), arg2Val),
cb.equal (root.get (keyAttr).get (numericAttr), sq)
));
results = em.createQuery (cq).getResultList ();
return results.size() == 0 ? null : results.get (0);
}
This is a code snippet that calls it:
Charge charge = DAOHelper.findWithUpperLimit (Charge.class, BigDecimal.class,
Charge_.key,
ChargeKey_.prodCd, invoice.getCharge().getChargeKey().getProdCd(),
ChargeKey_.optType, invoice.getCharge().getChargeKey().getOptType(),
ChargeKey_.optLevel, invoice.getCharge().getChargeKey().getOptType(),
em);
and here's the SQL that it generates:
select charge0_.OPTION_TYPE_CD as OPTION_1_50_,
charge0_.OPTION_LEVEL as OPTION_LEV2_50_,
charge0_.PROD_CD as PROD_CD3_50_,
charge0_.CHG_NU as CHG_NU4_50_
from CHARGE charge0_
where charge0_.PROD_CD=?
and charge0_.OPTION_TYPE_CD=?
and charge0_.OPTION_LEVEL=(select max(charge1_.OPTION_LEVEL)
from CHARGE charge1_
where charge1_.PROD_CD=?
and charge1_.OPTION_TYPE_CD=?
and charge1_.OPTION_LEVEL<=1358.00
)