@Override
    public StringExpression prepend(Expression<String> s) {
        if (s instanceof Constant<?>) {
            return prepend(((Constant<String>) s).getConstant());
        } else {
            return super.prepend(s);
        }
    }
StringConstant represents String constants

@author tiwe
querydsl-core/src/test/java/com/querydsl/core/StringConstant.java