add Session.getReference(Object)

like in Hibernate Reactive

this is really useful for dealing with detached instances + associations

hibernate-core/src/main/java/org/hibernate/Session.java

@@ -781,6 +781,18 @@ public interface Session extends SharedSessionContract, EntityManager {
 	 */
 	String getEntityName(Object object);
 
+	/**
+	 * Return the persistent instance with the same identity as the given instance, which
+	 * might be detached, assuming that the instance is still persistent in the database.
+	 * This method never results in access to the underlying data store, and thus might
+	 * return a proxy that must be initialized explicitly.
+	 *
+	 * @param object a detached persistent instance
+	 *
+	 * @return the persistent instance or proxy
+	 */
+	<T> T getReference(T object);
+	
 	/**
 	 * Create an {@link IdentifierLoadAccess} instance to retrieve the specified entity type by
 	 * primary key.

hibernate-core/src/main/java/org/hibernate/engine/spi/SessionDelegatorBaseImpl.java

@@ -900,6 +900,11 @@ public class SessionDelegatorBaseImpl implements SessionImplementor {
 		return delegate.getEntityName( object );
 	}
 
+	@Override
+	public <T> T getReference(T object) {
+		return delegate.getReference( object );
+	}
+
 	@Override
 	public <T> IdentifierLoadAccess<T> byId(String entityName) {
 		return delegate.byId( entityName );

hibernate-core/src/main/java/org/hibernate/internal/SessionImpl.java

@@ -1703,6 +1703,19 @@ public class SessionImpl
 		return entry.getPersister().getEntityName();
 	}
 
+	@Override @SuppressWarnings("unchecked")
+	public <T> T getReference(T object) {
+		checkOpen();
+		if ( object instanceof HibernateProxy ) {
+			LazyInitializer initializer = ( (HibernateProxy) object ).getHibernateLazyInitializer();
+			return (T) getReference( initializer.getPersistentClass(), initializer.getIdentifier() );
+		}
+		else {
+			EntityPersister persister = getEntityPersister( null, object );
+			return (T) getReference( persister.getMappedClass(), persister.getIdentifier(object, this) );
+		}
+	}
+
 	private void throwTransientObjectException(Object object) throws HibernateException {
 		throw new TransientObjectException(
 				"object references an unsaved transient instance - save the transient instance before flushing: " +