@Override
    protected boolean equalsExpression(ExpressionNode obj) {
        DocumentFieldNode rhs = (DocumentFieldNode)obj;
        if (!fieldName.equals(rhs.fieldName)) {
            return false;
        }
        return equals(result, rhs.result);
    }
Sets the name of the field whose value to retrieve.

@param fieldName The field name to set.
searchlib/src/main/java/com/yahoo/searchlib/expression/DocumentFieldNode.java