HHH-16955 - Better define how joins are handled with implicit Query select clause

hibernate-core/src/main/java/org/hibernate/query/hql/internal/SemanticQueryBuilder.java

@@ -1209,8 +1209,10 @@ public class SemanticQueryBuilder<R> extends HqlParserBaseVisitor<Object> implem
 
 	private void applyJoinsToInferredSelectClause(SqmFrom<?,?> sqm, SqmSelectClause selectClause) {
 		sqm.visitSqmJoins( (sqmJoin) -> {
-			selectClause.addSelection( new SqmSelection<>( sqmJoin, sqmJoin.getAlias(), creationContext.getNodeBuilder() ) );
+			if ( sqmJoin.isImplicitlySelectable() ) {
-			applyJoinsToInferredSelectClause( sqmJoin, selectClause );
+				selectClause.addSelection( new SqmSelection<>( sqmJoin, sqmJoin.getAlias(), creationContext.getNodeBuilder() ) );
+				applyJoinsToInferredSelectClause( sqmJoin, selectClause );
+			}
 		} );
 	}
 

hibernate-core/src/main/java/org/hibernate/query/sqm/tree/domain/SqmPluralPartJoin.java

@@ -58,6 +58,11 @@ public class SqmPluralPartJoin<O,T> extends AbstractSqmJoin<O,T> implements SqmQ
 		);
 	}
 
+	@Override
+	public boolean isImplicitlySelectable() {
+		return false;
+	}
+
 	@Override
 	public SqmPluralPartJoin<O, T> copy(SqmCopyContext context) {
 		final SqmPluralPartJoin<O, T> existing = context.getCopy( this );

hibernate-core/src/main/java/org/hibernate/query/sqm/tree/from/SqmAttributeJoin.java

@@ -24,6 +24,11 @@ public interface SqmAttributeJoin<O,T> extends SqmQualifiedJoin<O,T>, JpaFetch<O
 	@Override
 	SqmFrom<?,O> getLhs();
 
+	@Override
+	default boolean isImplicitlySelectable() {
+		return !isFetched();
+	}
+
 	@Override
 	SqmPathSource<T> getReferencedPathSource();
 

hibernate-core/src/main/java/org/hibernate/query/sqm/tree/from/SqmCrossJoin.java

@@ -55,6 +55,11 @@ public class SqmCrossJoin<T> extends AbstractSqmFrom<T, T> implements JpaCrossJo
 		this.sqmRoot = sqmRoot;
 	}
 
+	@Override
+	public boolean isImplicitlySelectable() {
+		return true;
+	}
+
 	@Override
 	public SqmCrossJoin<T> copy(SqmCopyContext context) {
 		final SqmCrossJoin<T> existing = context.getCopy( this );

hibernate-core/src/main/java/org/hibernate/query/sqm/tree/from/SqmCteJoin.java

@@ -61,6 +61,11 @@ public class SqmCteJoin<T> extends AbstractSqmQualifiedJoin<T, T> implements Jpa
 		this.cte = cte;
 	}
 
+	@Override
+	public boolean isImplicitlySelectable() {
+		return false;
+	}
+
 	@Override
 	public SqmCteJoin<T> copy(SqmCopyContext context) {
 		final SqmCteJoin<T> existing = context.getCopy( this );

hibernate-core/src/main/java/org/hibernate/query/sqm/tree/from/SqmDerivedJoin.java

@@ -85,6 +85,11 @@ public class SqmDerivedJoin<T> extends AbstractSqmQualifiedJoin<T, T> implements
 		return joinType;
 	}
 
+	@Override
+	public boolean isImplicitlySelectable() {
+		return false;
+	}
+
 	@Override
 	public SqmDerivedJoin<T> copy(SqmCopyContext context) {
 		final SqmDerivedJoin<T> existing = context.getCopy( this );

hibernate-core/src/main/java/org/hibernate/query/sqm/tree/from/SqmEntityJoin.java

@@ -59,6 +59,11 @@ public class SqmEntityJoin<T> extends AbstractSqmQualifiedJoin<T, T> implements
 		this.sqmRoot = sqmRoot;
 	}
 
+	@Override
+	public boolean isImplicitlySelectable() {
+		return true;
+	}
+
 	@Override
 	public SqmEntityJoin<T> copy(SqmCopyContext context) {
 		final SqmEntityJoin<T> existing = context.getCopy( this );

hibernate-core/src/main/java/org/hibernate/query/sqm/tree/from/SqmJoin.java

@@ -15,8 +15,16 @@ import org.hibernate.query.sqm.tree.SqmJoinType;
  * @author Steve Ebersole
  */
 public interface SqmJoin<O,T> extends SqmFrom<O,T> {
+	/**
+	 * The type of join - inner, cross, etc
+	 */
 	SqmJoinType getSqmJoinType();
 
+	/**
+	 * When applicable, whether this join should be included in an implicit select clause
+	 */
+	boolean isImplicitlySelectable();
+
 	@Override
 	<X, Y> SqmAttributeJoin<X, Y> join(String attributeName);
 

hibernate-core/src/test/java/org/hibernate/orm/test/query/joinfetch/JoinResultTests.java

@@ -0,0 +1,143 @@
+/*
+ * 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.joinfetch;
+
+import java.util.UUID;
+
+import org.hibernate.internal.util.StringHelper;
+
+import org.hibernate.testing.jdbc.SQLStatementInspector;
+import org.hibernate.testing.orm.domain.StandardDomainModel;
+import org.hibernate.testing.orm.domain.retail.Product;
+import org.hibernate.testing.orm.domain.retail.Vendor;
+import org.hibernate.testing.orm.junit.DomainModel;
+import org.hibernate.testing.orm.junit.Jira;
+import org.hibernate.testing.orm.junit.SessionFactory;
+import org.hibernate.testing.orm.junit.SessionFactoryScope;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests covering how joins and return-type affect the "shape" of the domain result
+ *
+ * @author Steve Ebersole
+ */
+@DomainModel(standardModels = StandardDomainModel.RETAIL)
+@SessionFactory(useCollectingStatementInspector = true)
+@Jira( "https://hibernate.atlassian.net/browse/HHH-16955" )
+public class JoinResultTests {
+	@Test
+	void testSimpleJoin(SessionFactoryScope scope) {
+		final SQLStatementInspector statementInspector = scope.getCollectingStatementInspector();
+
+		final String query = "from Product join vendor";
+
+		// supports `Product.class` - the single root
+		scope.inTransaction( (session) -> {
+			statementInspector.clear();
+			final Product product = session
+					.createQuery( query, Product.class )
+					.getSingleResult();
+			assertThat( product ).isNotNull();
+			// only the Product columns should be selected
+			assertThat( extractNumberOfSelections( statementInspector.getSqlQueries().get( 0 ) ) ).isEqualTo( 5 );
+		} );
+
+		// supports `Object[].class` - array[0] == the root && array[1] == the join
+		scope.inTransaction( (session) -> {
+			statementInspector.clear();
+			final Object[] result = session
+					.createQuery( query, Object[].class )
+					.getSingleResult();
+			assertThat( result ).isNotNull();
+			assertThat( result.length ).isEqualTo( 2 );
+			// both the Product and Vendor columns should be selected
+			assertThat( extractNumberOfSelections( statementInspector.getSqlQueries().get( 0 ) ) ).isGreaterThan( 5 );
+			assertThat( ( (Product) result[0] ).getVendor() ).isSameAs( result[1] );
+		} );
+	}
+
+	@Test
+	void testSimpleJoinFetch(SessionFactoryScope scope) {
+		final String query = "from Product join fetch vendor";
+
+		// supports `Product.class` - the single root
+		scope.inTransaction( (session) -> {
+			final Product product = session
+					.createQuery( query, Product.class )
+					.getSingleResult();
+			assertThat( product ).isNotNull();
+		} );
+
+		// supports `Object[].class` - array[0] == the root
+		scope.inTransaction( (session) -> {
+			final Object[] result = session
+					.createQuery( query, Object[].class )
+					.getSingleResult();
+			assertThat( result ).isNotNull();
+			assertThat( result.length ).isEqualTo( 1 );
+		} );
+	}
+
+	@Test
+	void testSimpleCrossJoin(SessionFactoryScope scope) {
+		final SQLStatementInspector statementInspector = scope.getCollectingStatementInspector();
+
+		final String query = "from Product cross join vendor";
+
+		// supports `Product.class` - the single root
+		scope.inTransaction( (session) -> {
+			statementInspector.clear();
+			final Product product = session
+					.createQuery( query, Product.class )
+					.getSingleResult();
+			assertThat( product ).isNotNull();
+			// both the Product and Vendor columns should be selected
+			assertThat( extractNumberOfSelections( statementInspector.getSqlQueries().get( 0 ) ) ).isEqualTo( 5 );
+		} );
+
+		// supports `Object[].class` - array[0] == the root && array[1] == the join
+		scope.inTransaction( (session) -> {
+			statementInspector.clear();
+			final Object[] result = session
+					.createQuery( query, Object[].class )
+					.getSingleResult();
+			assertThat( result ).isNotNull();
+			assertThat( result.length ).isEqualTo( 2 );
+			// both the Product and Vendor columns should be selected
+			assertThat( extractNumberOfSelections( statementInspector.getSqlQueries().get( 0 ) ) ).isGreaterThan( 5 );
+			assertThat( ( (Product) result[0] ).getVendor() ).isSameAs( result[1] );
+		} );
+	}
+
+	@BeforeEach
+	void createTestData(SessionFactoryScope scope) {
+		scope.inTransaction( (session) -> {
+			final Vendor vendor = new Vendor( 1, "ACME", "acme", "Some notes" );
+			final Product product = new Product( 1, UUID.randomUUID(), vendor );
+			session.persist( vendor );
+			session.persist( product );
+		} );
+	}
+
+	@AfterEach
+	void dropTestData(SessionFactoryScope scope) {
+		scope.inTransaction( (session) -> {
+			session.createMutationQuery( "delete Product" ).executeUpdate();
+			session.createMutationQuery( "delete Vendor" ).executeUpdate();
+		} );
+	}
+
+	private static int extractNumberOfSelections(String sql) {
+		final int fromClauseStartPosition = sql.indexOf( " from " );
+		final String selectClause = sql.substring( 0, fromClauseStartPosition );
+		return StringHelper.count( selectClause, "," ) + 1;
+	}
+}