Loading objects by queries

Often we are interested in a (sub)set of results. We populate our database with additional hibintro.v1.model.User instances: 

package hibintro.v1.run;
	...
	public class PersistUsers {
	...
	final Transaction transaction = session.beginTransaction();

	final User users[] = {new User("wings", "Fred Wings"),
        new User("eve", "Eve Briggs")} ;
	for (final User u : users ) {session.save(u);}

	transaction.commit(); ...

Now we'd like to retrieve these objects. Hibernate offers the Hibernate Query Language (HQL) for object queries. As we will see HQL extends SQL with respect to polymorphic queries. The current example does not use inheritance leaving us with a simple HQL query ❶ in hibintro.v1.run.RetrieveAll:

Figure 937. Retrieving hibintro.v1.model.User instances by HQL. 
package hibintro.v1.run;
	  ...
	  public class RetrieveAll {
	  ...
	  final Query searchUsers = session.createQuery("from User");❶
	  final List<User> users = (List<User>) searchUsers.list();
	  for (final User u: users) {
	  System.out.println("uid=" + u.getUid() + ", " + u.getCname());
	  }

Being used to SQLwe notice the absence of a SELECT clause in ❶: The ratio behind is having a focus on objects rather than on attribute sets. Thus our HQL query returns a set of hibintro.v1.model.User instances: 

uid=eve, Eve Briggs
uid=goik, Martin Goik
uid=wings, Fred Wings

exercise No. 17

HQL and SQL, analyzing results.

Q:

We may actually retrieve attributes rather than objects. For this purpose our query actually resembles standard SQL ❶: 

final Query searchUsers = session.createQuery("select uid, cname from User" ❶);
		final Object queryResult ❷= searchUsers.list();

Use the Class.getSimpleName() reflection method to iteratively analyze the queryResult ❷ instance's structure. This guides you in finding suitable casts to add code similar as in Figure 937, “Retrieving hibintro.v1.model.User instances by HQL. ” in order to write user's attribute values to standard output.

A:

A possible implementation reads:

package hibintro.v1.run;
		...
		public class GetUsersAsAttributes {
		...
		final Query searchUsers = session.createQuery("select uid, cname from User");

		@SuppressWarnings("unchecked")
		final Object queryResult = searchUsers.list();
		System.out.println("queryResult type:" + queryResult.getClass().getSimpleName()); ❶
		final List<Object> usersAttributes = (List<Object>) queryResult;
		for (final Object o: usersAttributes) {
		System.out.println("result set element type:" + o.getClass().getSimpleName()); ❷
		final Object attributes[] = (Object []) o;
		for (Object attribute: attributes) {
		System.out.println("attribute value:" + attribute);
		}
		}...

Actually the two lines ❶ and ❷ are only needed during the development process to discover the result set's object structure.

The careful reader may already expect HQL to offer additional features namely predicate based queries. Following hibintro.v1.run.SelectUser we may restrict our result set by an SQL style WHERE clause: 

    final List<User> users = (List<User>) session.createQuery(
        "from User u where u.cname like '%e%'").list();
	for (final User u: users) {
	System.out.println("Found user '" + u.getCname() + "'");
	}

This time we receive a true subset of hibintro.v1.model.User instances: 

Found user 'Eve Briggs'
	Found user 'Fred Wings'
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