Merge Data from Multiple List<SObject> for use in Update/Insert

Question

I'm attempting to rollup some Activity data to Campaign Members from a Trigger event. I've already worked through my trigger/filtering logic and now I have a list of affected campaign members:

List<CampaignMember> campaign_members = CampaignUtils.listCampaignMembersByWhoId(whoId_list); /* 1 SOQL */

As well as a list of Recent Activities per Lead/Contact (which is the WhoId):

List<ActivityHistory> recent_activities = CampaignUtils.fetchRecentActivityForCampaginMembers(campaign_members,true);

*Example Instance: 20+ activities are updated, between 3 Leads/Contacts, producing 6 Campaign Members affected =List<CampaignMember> campaign_members, as well as 3 recent activities, (1 per Who [ContactId/LeadId]) =List<ActivityHistory> recent_activities

So with these 2 list I need to merge or update the campaign_members fields with data in the recent_activities. For example:

campaign_members.F2F_Activity_Date__c = recent_activities.ActivityDate

BUT, this is based on matching WhoId(s). I'm thinking I need a loop of some sort but not sure how to say, for each campaign_members find the matching recent_activity, and then add that activity data to this member.

Thanks in advance for any help!

EDIT: My current attempt looks something like this

for (CampaignMember member : campaign_members) {
    for(ActivityHistory act : recent_activities){
        if (act.WhoId == member.LeadId || act.WhoId == member.ContactId) {
            campaign_members.get(**THIS MEMBER**).F2F_Activity_ID__c = act.Id;
            campaign_members.get(member).custom_field__c = act.value;
        }
    }
}
update campaign_members;

Answer 1

In your example you provided, you are very close. The only thing you need to understand is that in the Java world (and therefore the Apex world), any non-primitive data types are pass-by-reference. Your example will look something like this to work:

Map activityMap = new Map();
for (ActivityHistory act: recent_Activities) {
  activityMap.put(act.WhoId,act);
}

for (CampaignMember member : campaign_members) {
    if(activityMap.containsKey(member.ContactId)){
        member.F2F_Activity_ID__c = activityMap.get(member.ContactId).Id;
        member.custom_field__c = activityMap.get(member.ContactId).value;
    }
}
update campaign_members;

In addition to fix the issue you will also want to prevent the nested for loops. You'll see how I used a Map to prevent that.

Answer 2

You should avoid nested loops and use a Map instead. Since WhoId is not unique among your ActivityHistory records, you should ideally use Map<Id, List<ActivityHistory>>, but in this case it looks like you will arbitrarily pick which ActivityHistory to use, so Map<Id, ActivityHistory will probably suffice.

static void updateCampaignMembers(List<CampaignMember> campaign_members, List<ActivityHistory> recent_activities)
{
    Map<Id, ActivityHistory> whoIdToHistory = new Map<Id, ActivityHistory>();
    for (ActivityHistory history : recent_activities)
    {
        whoIdToHistory.put(history.WhoId, history);
    }

    for (CampaignMember member : campaign_members)
    {
        ActivityHistory leadMatch = whoIdToHistory.get(member.LeadId);
        ActivityHistory contactMatch = whoIdToHistory.get(member.ContactId);
        if (leadMatch != null) mergeValues(member, leadMatch);
        if (contactMatch != null) mergeValues(member, contactMatch);
    }
    update campaign_members;
}

static void mergeValues(CampaignMember member, ActivityHistory history)
{
    if (history == null) return;
    member.F2F_Activity_ID__c = history.Id;
    member.custom_field__c = history.value;
}