@Override
    public Object visit(Constant<?> expr, Void context) {
        Object value = super.visit(expr, context);
        return morphia.getMapper().toMongoObject(null, null, value);
    }
{@code MorphiaSerializer} extends {@link MongodbSerializer} with Morphia specific annotation handling

@author tiwe
querydsl-mongodb/src/main/java/com/querydsl/mongodb/morphia/MorphiaSerializer.java