What causes "Cannot use identity column key generation with mapping for: %s" in hibernate/hibernate-orm?

Switch to a shared generator: @GeneratedValue(strategy = SEQUENCE) with @SequenceGenerator on the root

hibernate/hibernate-orm · error · MappingException

Cannot use identity column key generation with <union-subcla

Error message

Cannot use identity column key generation with <union-subclass> mapping for: %s

What it means

TABLE_PER_CLASS inheritance (union-subclass) answers polymorphic queries with UNION over the concrete class tables, which cannot produce meaningful results if each table independently assigns primary keys via the database IDENTITY mechanism. UnionSubclassEntityPersister.validateGenerator() therefore rejects IdentityGenerator at SessionFactory boot with a MappingException naming the entity.

Source

Thrown at hibernate-core/src/main/java/org/hibernate/persister/entity/UnionSubclassEntityPersister.java:219

						getIdentifierMapping(),
						tableName,
						getIdentifierColumnNames()
				)
		);

		visitAttributeMappings( (attribute) -> {
			builder.addAttribute( attribute );
			attribute.forEachSelectable( (selectableIndex, selectable)
					-> builder.addColumn( attribute, ColumnDescriptor.from( selectable ) ) );
		} );

		// Union subclass has only one table, so entity-wide flag equals table flag
		return new EntityTableDescriptor[] { builder.build( builder.isSelfReferential ) };
	}

	protected void validateGenerator() {
		if ( getGenerator() instanceof IdentityGenerator ) {
			throw new MappingException( "Cannot use identity column key generation with <union-subclass> mapping for: " + getEntityName() );
		}
	}

	@Override
	public boolean containsTableReference(String tableExpression) {
		for ( String subclassTableExpression : subclassTableExpressions ) {
			if ( subclassTableExpression.equals( tableExpression ) ) {
				return true;
			}
		}
		return false;
	}


	@Override
	public UnionTableReference createPrimaryTableReference(
			SqlAliasBase sqlAliasBase,
			SqlAstCreationState creationState) {

View on GitHub (pinned to fad1729dce)

Solutions

Switch to a shared generator: @GeneratedValue(strategy = SEQUENCE) with @SequenceGenerator on the root
Use a TABLE generator, UUID strategy, or assigned identifiers if the database has no sequences
If IDENTITY columns are mandatory, switch the hierarchy to JOINED inheritance instead
Avoid generator class "native" on identity-only dialects - pin the strategy explicitly

Example fix

// before
@Entity
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public class Vehicle {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY) // rejected
    Long id;
}

// after
@Entity
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public class Vehicle {
    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "veh_seq")
    @SequenceGenerator(name = "veh_seq", sequenceName = "vehicle_seq", allocationSize = 50)
    Long id;
}

Defensive patterns

Strategy: validation

Validate before calling

// fail fast in CI: TABLE_PER_CLASS + IDENTITY is rejected by Hibernate
for (Class<?> e : entityClasses) {
    Inheritance inh = e.getAnnotation(Inheritance.class);
    GeneratedValue gv = e.getAnnotation(GeneratedValue.class);
    if (inh != null && inh.strategy() == InheritanceType.TABLE_PER_CLASS
            && gv != null && gv.strategy() == GenerationType.IDENTITY) {
        throw new IllegalStateException("TABLE_PER_CLASS + IDENTITY not supported: " + e.getName());
    }
}

Try / catch

try { sessionFactory = cfg.buildSessionFactory(); } catch (MappingException e) { if (e.getMessage().contains("identity column key generation")) { /* switch the hierarchy to SEQUENCE/TABLE/UUID generation */ } throw e; }

Prevention

Default new hierarchies to SEQUENCE generation; reserve IDENTITY for non-inherited tables
Never rely on generator class 'native' for union-subclass mappings on identity-only databases
Add a config-scan test for the TABLE_PER_CLASS + IDENTITY combination

When it happens

Trigger: @Inheritance(strategy = TABLE_PER_CLASS) with @GeneratedValue(strategy = GenerationType.IDENTITY) on the root identifier; hbm.xml <union-subclass> with <generator class="native"/> on a dialect where native resolves to identity (MySQL, SQL Server); dialect-native generation picking identity implicitly.

Common situations: Porting an app from SINGLE_TABLE to TABLE_PER_CLASS on MySQL where auto-increment was used; generated mappings defaulting to IDENTITY; using database auto-increment columns out of habit on union hierarchies.

Related errors

AI-assisted analysis of hibernate/hibernate-orm@fad1729dce (2026-08-22). Data as JSON: /api/errors/01a5758ae30a0989. Report an issue: GitHub.