trigger on contact for counting of activities history and also updated on Account and opportunity object

Question

field for number of Activities for each Contact, regardless of Type or Status. Full Details: The requirement is to be able to create reports based on Contacts or account or Opportunities and see a “Summary” field for Total # of Activities associated with either Contacts, Accounts or Opportunities. Note that we do not want to see a line for each activity, we want to see one line for the each contact or opp or account with the activity summary count. Note: The original request also included the ability to have a unique activity count on reports

Answer 1

I haven't tested this code, but you could do something like this for an insert (you'd need to also cover updates and deletes). In this example, NumberOfActivites__c is your custom Task count field on the Contact object:

Map<Id,Integer> countMap = new Map<Id,Integer>();
List<Contact> contactList = new List<Contact>();

for (Task t : trigger.new){
    //get id's of all contacts affected by the batch
    Id w = t.whoId;
    if (w.getSObjectType().getDescribe().getName() == 'Contact'){
        //since there could be more than one task related to a contact 
        //in a batch, you would have to count them
        if (countMap.keyset().containts(w)){
            countMap.get(w) += 1;
        } else {
            countMap.put(w,1);
        }
    }
}

//get list of contacts to be updated
contactList = [Select Id, NumberOfActivities__c
               From Contact
               Where Id In :countMap.keyset()];

//modify contacts in list with new count
for (Contact c : contactList){
    c.NumberOfActivites__c = c.NumberOfActivites__c + countMap.get(c.Id));
}

//do the update
update contactList;