APEX Trigger doesn't actually do what I want it to do

Question

Basically, I just want this trigger to update a custom number field (by incrementing it) whenever an email is sent or received from a case.

Here's the trigger:

trigger updateFieldUponEmail on EmailMessage (before insert) {
        Set<ID> caseIDs = new Set<ID>();

        for(EmailMessage msg : trigger.new){
            if(msg.parentID.getsObjectType() == Case.sObjectType)
               caseIDs.add(msg.parentID);
        }

        for(Case c: [SELECT timesUpdated__c FROM Case WHERE Id IN :CaseIDs]){
            c.timesUpdated__c++;
        }
}

Validates fine, deploys fine. It just doesn't actually increment that field. Not sure why. I tested by sending an email from a case. Am I testing wrong or is the code wrong?

Answer 1

trigger updateFieldUponEmail on EmailMessage (before insert) {
    Set<ID> caseIDs = new Set<ID>();

    for(EmailMessage msg : trigger.new){
        if(msg.parentID.getsObjectType() == Case.sObjectType)
           caseIDs.add(msg.parentID);
    }

    List<Case> casesToUpdate = [SELECT timesUpdated__c FROM Case WHERE Id IN :caseIDs];

    for(Case thisCase: casesToUpdate){
        thisCase.timesUpdated__c++;
    }

    update casesToUpdate;
}

Answer 2

To address @crop1645's points and also to avoid an unnecessary query of Case when the email relates to other objects:

trigger updateFieldUponEmail on EmailMessage (before insert) {

    Map<Id, Integer> m = new Map<Id, Integer>();
    for(EmailMessage msg : trigger.new){
        if (msg.parentID.getsObjectType() == Case.sObjectType) {
            Integer n = m.get(msg.parentID);
            m.put(msg.parentID, (n != null ? n : 0) + 1);
        }
    }

    if (m.size() > 0) {
        List<Case> updates = [
                SELECT Id, timesUpdated__c
                FROM Case
                WHERE Id IN :m.keySet()
                ];
        for (Case c: updates){
            c.timesUpdated__c = (c.timesUpdated__c != null ? c.timesUpdated__c : 0)
                    + m.get(c.Id);  
        }
        update updates;
    }
}