I am trying to get back a count of records in the task / event object for contacts so that I can update a contact field with the correct vale of activity.

I am passing a LIST<ID> to the function. The trouble appears on the highlighted lines below

LIST<TASK> objTasks = [SELECT WHOID, count(ID) FROM TASK WHERE WHOID IN :ContactIDs];
LIST<EVENT> objEvents = [SELECT WHOID, count(ID) FROM EVENT WHERE WHOID IN :ContactIDs];

The Error I get is:

Field must be grouped or aggregated: WhoId

But if I try and group it, it gives me errors as well

LIST<TASK> objTasks = [SELECT WHOID, count(ID) FROM TASK WHERE WHOID IN :ContactIDs GROUP BY WHOID];
LIST<EVENT> objEvents = [SELECT WHOID, count(ID) FROM EVENT WHERE WHOID IN :ContactIDs GROUP BY WHOID];

This gives the error

Illegal assignment from List<AggregateResult> to List<Task>

 

The Function

PUBLIC VOID updateActivityCount( LIST<ID> listContacts )
	{
        LIST<ID> ContactIDs = NEW LIST<ID>();
		ContactIDs = listContacts;

		INTEGER tmpTasks = 0;
		INTEGER tmpEvents = 0;

		
		LIST<CONTACT> contactUpdate = NEW LIST<CONTACT>();
		MAP<ID, INTEGER> contactTasks = NEW MAP<ID, INTEGER>();
		MAP<ID, INTEGER> contactEvents = NEW MAP<ID, INTEGER>();

        LIST<TASK> objTasks = [SELECT WHOID, count(ID) FROM TASK WHERE WHOID IN :ContactIDs];
        LIST<EVENT> objEvents = [SELECT WHOID, count(ID) FROM EVENT WHERE WHOID IN :ContactIDs];

		FOR(TASK thisTask : objTasks)
		{	contactTasks.put(thisTask[0], thisTask[1]);	}

		FOR(EVENT thisEvent : objEvents)
		{	contactEvents.put(thisEvent[0], thisEvent[1]);	}

		LIST<CONTACT> objContacts = [SELECT ID FROM Contact WHERE ID IN :ContactIDs];
		FOR(CONTACT thisContact : objContacts)
		{
			IF( contactTasks.containsKey( thisContact.Id ) )
			{	tmpTasks = contactTasks.get( thisContact.Id );	}
			ELSE
			{	tmpTasks = 0;	}

			IF( contactEvents.containsKey( thisContact.Id ) )
			{	tmpTasks = contactEvents.get( thisContact.Id );	}
			ELSE
			{	tmpEvents = 0;	}

			thisContact.activity_this_cfy__c = tmpTasks + tmpEvents;
			contactUpdate.add( thisContact );
		}

		//--    Execute the update for contactUpdate
		//------------------------------------------------------------------------------------------------------------------------------
		if ( !contactUpdate.isEmpty() )
		{
			TRY{ update contactUpdate; }
			CATCH ( Exception ex )
			{   SYSTEM.DEBUG(thisPage + 'Could not update Contact. Cause: ' + ex.getCause() );  }
		}
	}	//--	END updateActivityCount