Hibernate: LazyInitializationException: failed to lazily initialize a collection of role. Could not initialize proxy - no Session

It seems that model is a detached entity.

Try to merge and perform operations on a merge instance:

@Override
public void process(Model model) {
     ...
    Model mergedModel = session.merge(model);

    mergedModel.addEntity(createEntity());
    ...
}

So as @Maciej Kowalski mentioned after first @Transactional read of my model it's already in deatached state and call to get entities from another @Transactional method failed with LazyInitializationException.

I've changed my service a bit to get model from database in the same transaction:

@Service
@Transactional
class ServiceImpl implements Service {
    @Override
    public void process(long modelId) {
        ...
        Model model = modelDao.get(modelId);
        model.addEntity(createEntity());
        ...
    }
}

Now everything works as expected.