HHH-18815 add Generator.allowMutation()

Signed-off-by: Gavin King <gavin@hibernate.org>

hibernate-core/src/main/java/org/hibernate/annotations/Generated.java

@@ -97,8 +97,12 @@ public @interface Generated {
 	/**
 	 * Determines if the value currently assigned to the annotated property
 	 * is included in SQL {@code insert} and {@code update} statements. This
-	 * is useful if the generated value is obtained by transforming the
+	 * is useful if:
-	 * assigned property value as it is being written.
+	 * <ul>
+	 * <li>the generated value is obtained by transforming the assigned
+	 *     property value as it is being written, or
+	 * <li>assigning a value disables generation of a value.
+	 * </ul>
 	 * <p>
 	 * Often used in combination with {@link SQLInsert}, {@link SQLUpdate},
 	 * or {@link ColumnTransformer#write()}.

hibernate-core/src/main/java/org/hibernate/generator/Generator.java

@@ -113,6 +113,7 @@ public interface Generator extends Serializable {
 	 * @see #generatedOnExecution()
 	 * @see BeforeExecutionGenerator
 	 * @see OnExecutionGenerator
+	 *
 	 * @since 6.4
 	 */
 	default boolean generatedOnExecution(Object entity, SharedSessionContractImplementor session) {
@@ -143,6 +144,19 @@ public interface Generator extends Serializable {
 		return false;
 	}
 
+	/**
+	 * Determine if this generator allows generated fields to be manually assigned a value on
+	 * {@linkplain #getEventTypes events} which do <em>not</em> trigger value generation.
+	 *
+	 * @return {@code true} if this generator allows manually assigned values,
+	 *         {@code false} otherwise (default).
+	 *
+	 * @since 7.0
+	 */
+	default boolean allowMutation() {
+		return false;
+	}
+
 	default boolean generatesSometimes() {
 		return !getEventTypes().isEmpty();
 	}

hibernate-core/src/main/java/org/hibernate/generator/internal/GeneratedGeneration.java

@@ -4,6 +4,7 @@
  */
 package org.hibernate.generator.internal;
 
+import org.hibernate.AnnotationException;
 import org.hibernate.annotations.Generated;
 import org.hibernate.dialect.Dialect;
 import org.hibernate.engine.spi.SharedSessionContractImplementor;
@@ -41,7 +42,10 @@ public class GeneratedGeneration implements OnExecutionGenerator, BeforeExecutio
 	public GeneratedGeneration(Generated annotation) {
 		eventTypes = fromArray( annotation.event() );
 		sql = isEmpty( annotation.sql() ) ? null : new String[] { annotation.sql() };
-		writable = annotation.writable() || sql != null;
+		writable = annotation.writable();
+		if ( sql != null && writable ) {
+			throw new AnnotationException( "A field marked '@Generated(writable=true)' may not specify explicit 'sql'" );
+		}
 	}
 
 	@Override
@@ -51,7 +55,9 @@ public class GeneratedGeneration implements OnExecutionGenerator, BeforeExecutio
 
 	@Override
 	public boolean referenceColumnsInSql(Dialect dialect) {
-		return writable;
+		// include the column in when the field is writable,
+		// or when there is an explicit SQL expression
+		return writable || sql != null;
 	}
 
 	@Override
@@ -61,7 +67,9 @@ public class GeneratedGeneration implements OnExecutionGenerator, BeforeExecutio
 
 	@Override
 	public boolean writePropertyValue() {
-		return writable && sql==null;
+		// include a ? parameter when the field is writable,
+		// but there is no explicit SQL expression
+		return writable;
 	}
 
 	@Override
@@ -71,13 +79,15 @@ public class GeneratedGeneration implements OnExecutionGenerator, BeforeExecutio
 
 	@Override
 	public boolean generatedOnExecution(Object entity, SharedSessionContractImplementor session) {
-		if ( !writable ) {
+		if ( writable ) {
-			return true;
-		}
-
 			// When this is the identifier generator and writable is true, allow pre-assigned identifiers
 			final EntityPersister entityPersister = session.getEntityPersister( null, entity );
-		return entityPersister.getGenerator() != this || entityPersister.getIdentifier( entity, session ) == null;
+			return entityPersister.getGenerator() != this
+				|| entityPersister.getIdentifier( entity, session ) == null;
+		}
+		else {
+			return true;
+		}
 	}
 
 	@Override
@@ -91,4 +101,10 @@ public class GeneratedGeneration implements OnExecutionGenerator, BeforeExecutio
 	public boolean allowAssignedIdentifiers() {
 		return writable;
 	}
+
+	@Override
+	public boolean allowMutation() {
+		// the user may specify @Immutable if mutation should be disallowed
+		return true;
+	}
 }

hibernate-core/src/main/java/org/hibernate/tuple/entity/EntityMetamodel.java

@@ -321,15 +321,12 @@ public class EntityMetamodel implements Serializable {
 				}
 				else {
 					generators[i] = generator;
-					final boolean noParameter = generatedWithNoParameter( generator );
+					if ( !generator.allowMutation() ) {
-					if ( noParameter && !generator.allowAssignedIdentifiers() ) {
 						propertyInsertability[i] = false;
 						propertyUpdateability[i] = false;
 					}
 					if ( generator.generatesOnInsert() ) {
-						if ( noParameter ) {
+						propertyInsertability[i] = !generatedWithNoParameter( generator );
-							propertyInsertability[i] = false;
-						}
 						if ( generator.generatedOnExecution() ) {
 							foundPostInsertGeneratedValues = true;
 							if ( generator instanceof BeforeExecutionGenerator ) {
@@ -341,9 +338,7 @@ public class EntityMetamodel implements Serializable {
 						}
 					}
 					if ( generator.generatesOnUpdate() ) {
-						if ( noParameter ) {
+						propertyUpdateability[i] = !generatedWithNoParameter( generator );
-							propertyInsertability[i] = false;
-						}
 						if ( generator.generatedOnExecution() ) {
 							foundPostUpdateGeneratedValues = true;
 							if ( generator instanceof BeforeExecutionGenerator ) {