AggregateResults Grouping of Picklist to String Error

Question

I'm attempting to summarize invoice data by type of invoice grouped by time period. Income_Class__c is a picklist field and upon running this code I get an error: Invalid conversion from runtime type integer to string.

1 - What do I need to modify here to get the correct Income_Class__c picklist value to display?

2 - When this runs I get more results than desired. Any idea how I can make this only create records grouped by Income Class and Month (but also display the calendar year?).

AggregateResult[] RevDataMonth = [SELECT GROUPING(Income_Class__c)         IncomeGroup, CALENDAR_MONTH(Transaction_Date__c) CalMonth, 
                                CALENDAR_YEAR(Transaction_Date__c) CalYear, SUM(Net_Revenue__c) SumRev
                                FROM Invoice__c
                                WHERE Company__c = '001Q000000VDkpq'
                                GROUP BY ROLLUP(Income_Class__c, CALENDAR_MONTH(Transaction_Date__c), CALENDAR_YEAR(Transaction_Date__c))];
{
for (AggregateResult a : RevDataMonth){
        RevRollupAug12__c r = new RevRollupAug12__c();
        r.month__c = (Integer)a.get('CalMonth');
        r.year__c = (Integer)a.get('CalYear');
        r.Amount__c = (Decimal)a.get('SumRev');
        r.company__c = '001Q000000VDkpq'; 
        r.Time_Period__c = 'Month';
        r.Income_Class__c = (String)a.getvalue('IncomeGroup');
        insert r; }}}

Answer 1

The GROUPING() function is used to identify subtotal rows - it identifies subtotal rows in a result set, and returns 1 if the row is a subtotal for the field, and 0 otherwise - it doesn't yield the field value itself.

If you want the sum of the revenue for each combination of calendar month, year and income class, I think the SOQL query is a bit simpler:

AggregateResult[] RevDataMonth = 
   [SELECT CALENDAR_MONTH(Transaction_Date__c) CalMonth, 
        CALENDAR_YEAR(Transaction_Date__c) CalYear, 
        SUM(Net_Revenue__c) SumRev, Income_Class__c 
    FROM Invoice__c 
    WHERE Company__c = '001Q000000VDkpq'
    GROUP BY CALENDAR_MONTH(Transaction_Date__c), 
        CALENDAR_YEAR(Transaction_Date__c), Income_Class__c];
for (AggregateResult a : RevDataMonth) {
    RevRollupAug12__c r = new RevRollupAug12__c();
    r.month__c = (Integer)a.get('CalMonth');
    r.year__c = (Integer)a.get('CalYear');
    r.Amount__c = (Decimal)a.get('SumRev');
    r.company__c = '001Q000000VDkpq'; 
    r.Time_Period__c = 'Month';
    r.Income_Class__c = (String)a.get('Income_Class__c');
    insert r; 
}

Answer 2

I don't think you can do a simple casting from a numeric type to String. Have you tried this:

r.Income_Class__c = String.valueOf(a.getvalue('IncomeGroup'));