Fix Criteria Subquery#getSelection() returns null when it has not any select expression

hibernate-core/src/main/java/org/hibernate/query/sqm/tree/select/SqmSubQuery.java

@@ -127,6 +127,10 @@ public class SqmSubQuery<T> extends AbstractSqmSelectQuery<T> implements SqmSele
 
 	@Override
 	public SqmExpression<T> getSelection() {
+		final SqmSelectClause selectClause = getQuerySpec().getSelectClause();
+		if ( selectClause == null ) {
+			return null;
+		}
 		return this;
 	}
 

hibernate-core/src/test/java/org/hibernate/orm/test/query/criteria/RootEntityTypeModelTest.java

@@ -1,3 +1,9 @@
+/*
+ * Hibernate, Relational Persistence for Idiomatic Java
+ *
+ * License: GNU Lesser General Public License (LGPL), version 2.1 or later
+ * See the lgpl.txt file in the root directory or http://www.gnu.org/licenses/lgpl-2.1.html
+ */
 package org.hibernate.orm.test.query.criteria;
 
 import org.hibernate.testing.orm.junit.DomainModel;

hibernate-core/src/test/java/org/hibernate/orm/test/query/criteria/SubqueryGroupByTest.java

@@ -0,0 +1,145 @@
+package org.hibernate.orm.test.query.criteria;
+
+import java.util.List;
+
+import org.hibernate.testing.orm.junit.EntityManagerFactoryScope;
+import org.hibernate.testing.orm.junit.Jpa;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import jakarta.persistence.Entity;
+import jakarta.persistence.Id;
+import jakarta.persistence.ManyToOne;
+import jakarta.persistence.Table;
+import jakarta.persistence.criteria.CriteriaBuilder;
+import jakarta.persistence.criteria.CriteriaQuery;
+import jakarta.persistence.criteria.Root;
+import jakarta.persistence.criteria.Subquery;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+@Jpa(
+		annotatedClasses = {
+				SubqueryGroupByTest.JobTile.class,
+				SubqueryGroupByTest.Person.class,
+		}
+)
+public class SubqueryGroupByTest {
+
+	@BeforeEach
+	public void setUp(EntityManagerFactoryScope scope) {
+		scope.inTransaction(
+				entityManager -> {
+					JobTile worker = new JobTile( 1, "Worker" );
+					JobTile plumber = new JobTile( 2, "Plumber" );
+					Person p1 = new Person( 1, "Luigi", worker );
+					Person p2 = new Person( 2, "Andrea", plumber );
+
+					entityManager.persist( worker );
+					entityManager.persist( plumber );
+					entityManager.persist( p1 );
+					entityManager.persist( p2 );
+				}
+		);
+	}
+
+	@Test
+	public void subqueryGroupTest(EntityManagerFactoryScope scope) {
+
+		scope.inEntityManager(
+				entityManager -> {
+					final CriteriaBuilder criteriaBuilder = entityManager.getCriteriaBuilder();
+
+					final CriteriaQuery<Person> criteriaQuery = criteriaBuilder.createQuery( Person.class );
+					final Root<Person> person = criteriaQuery.from( Person.class );
+
+					criteriaQuery.select( person );
+
+					final Subquery<String> subquery = criteriaQuery.subquery( String.class );
+					assertEquals( 0, subquery.getGroupList().size() );
+
+					assertNull( subquery.getSelection() );
+
+					final Root<JobTile> jobTitle = subquery.from( JobTile.class );
+					subquery.select( jobTitle.get( "name" ) );
+					assertNotNull( subquery.getSelection() );
+
+					subquery.where( criteriaBuilder.equal( jobTitle.get( "name" ), "Worker" ) );
+
+					subquery.groupBy( jobTitle.get( "name" ) );
+					assertEquals( 1, subquery.getGroupList().size() );
+
+					criteriaQuery.where( person.get( "jobTile" ).get( "name" ).in( subquery ) );
+
+					List<Person> result = entityManager.createQuery( criteriaQuery ).getResultList();
+
+					assertEquals( 1, result.size() );
+					assertEquals( "Luigi", result.get( 0 ).getName() );
+
+				}
+		);
+	}
+
+	@Entity(name = "Person")
+	@Table(name = "PERSON_TABLE")
+	public static class Person {
+
+		@Id
+		private Integer id;
+
+		private String name;
+
+		@ManyToOne
+		private JobTile jobTile;
+
+		public Person() {
+		}
+
+		public Person(Integer id, String name, JobTile jobTile) {
+			this.id = id;
+			this.name = name;
+			this.jobTile = jobTile;
+		}
+
+		public Integer getId() {
+			return id;
+		}
+
+		public String getName() {
+			return name;
+		}
+
+		public JobTile getJobTile() {
+			return jobTile;
+		}
+	}
+
+	@Entity(name = "JobTile")
+	@Table(name = "JOBTITLE_TABLE")
+	public static class JobTile {
+
+		@Id
+		private Integer id;
+
+		private String name;
+
+		public JobTile() {
+		}
+
+		public JobTile(Integer id, String name) {
+			this.id = id;
+			this.name = name;
+		}
+
+		public Integer getId() {
+			return id;
+		}
+
+		public String getName() {
+			return name;
+		}
+	}
+
+}