JDO Typesafe Queries : Part 3 – Examples

In two previous blog posts, we introduced the idea of having a typesafe refactor friendly API for queries for JDO, and then described typical expression types necessary to achieve that. In this blog post we take that a step further, particularly now that DataNucleus SVN has a proof of concept query API to fulfil this requirement.

In these examples we have a class Product. Consequently we have a “query” class autogenerated – QProduct. Here is the QProduct class

public class QProduct extends PersistableExpressionImpl implements PersistableExpression
{
    public static QProduct candidate() {...} // candidate "this"
    public static QProduct candidate(String name) {...}
    public static QProduct parameter(String name) {...}
    public static QProduct variable(String name) {...}
    public NumericExpression id;
    public StringExpression name;
    public NumericExpression value;
    ...
}

So, as you can see we have access to the persistable fields/properties of Product by way of public fields in its query class. Now on to some examples

Filter only
Here we select all Products that have a value less than 40.0

TypesafeQuery tq = pm.newTypesafeQuery(Product.class);
QProduct cand = QProduct.candidate();
tq.filter(cand.value.lt(40.0));
List results = tq.executeList();

Filter + Order
Here we select all Products that have a value less than 40.0 and ordering by their name

TypesafeQuery tq = pm.newTypesafeQuery(Product.class);
QProduct cand = QProduct.candidate();
tq.filter(cand.value.lt(40.0)).orderBy(cand.name.asc());
List results = tq.executeList();

Filter + Order + Result
Here we select the product name and product value for all Products that have a value less than 40.0 , ordering by their name

TypesafeQuery tq = pm.newTypesafeQuery(Product.class);
QProduct cand = QProduct.candidate();
tq.filter(cand.value.lt(40.0)).orderBy(cand.name.asc());
List results = tq.executeResultList(true, cand.name, cand.value);

Filter with Methods
Here we select all Products that have a value less than 40.0 and name starting with “Wal”, and ordering by their name

TypesafeQuery tq = pm.newTypesafeQuery(Product.class);
QProduct cand = QProduct.candidate();
tq.filter(cand.value.lt(40.0).and(cand.name.startsWith("Wal")));
List results = tq.executeList();

Filter, using Parameters
Here we select all Products that have a value less than 40.0, using a parameter

TypesafeQuery tq = pm.newTypesafeQuery(Product.class);
QProduct cand = QProduct.candidate()
tq.filter(cand.value.lt(tq.doubleParameter("param1"))).setParameter("param1", 40.0);
List results = tq.executeList();

Order + Range
Here we select Products, ordering by the name and restrict to the first two

TypesafeQuery tq = pm.newTypesafeQuery(Product.class);
QProduct cand = QProduct.candidate();
tq.orderBy(cand.name.asc()).range(0,2);
List results = tq.executeList();

Filter with Variable
Here we select Inventory (which has a collection of Products) that contain a Product with a particular name

TypesafeQuery tq = pm.newTypesafeQuery(Inventory.class);
QInventory cand = QInventory.candidate();
QProduct var = QProduct.variable("var");
tq.filter(cand.products.contains(var).and(var.name.startsWith("Wal")));
List results = tq.executeList();

Filter with Subquery
Here we select Products with a value less than the average value of any Product

TypesafeQuery tq = pm.newTypesafeQuery(Product.class);
QProduct cand = QProduct.candidate();
TypesafeSubquery tqsub = tq.subquery(Product.class, "p");
QProduct candsub = QProduct.candidate("p");
tq.filter(cand.value.lt(tqsub.select(candsub.value.avg())));
List results = tq.executeList();

As you can see from the above queries we have no hard-coded class or field names, providing better refactorability. We can obviously make use of all JDOQL supported methods in the above queries, and define parameters and variables just like in the current JDOQL API.

It should be noted that this JDOQL facility is much more elegant and requires less code than the equivalent queries using JPA “Criteria”.