@Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        for(AttributeNode.Item item : attribute.getItems()) {
            sb.append(item.getName()).append(".");
        }
        sb.deleteCharAt(sb.length() - 1);
        return sb + ":" + comparison.getOperator() + now;
    }
Represents a query containing a valid now() expression. The now expression
is very strict right now, but can be expanded later.

@author Ulf Lilleengen
document/src/main/java/com/yahoo/document/select/convert/NowQueryExpression.java