trigger LaatsteCallMobileMiles on Task

/*
 * @see https://success.salesforce.com/answers?id=90630000000hdCVAAY
 */
trigger LaatsteCallMobileMiles on Task (after insert) {

    // Retrieve the email addresses of CreatedBy users for tasks currently
    // being processed by the trigger

    List<Task> newTasks = [
        SELECT Id, WhatId, Subject, CreatedDate, CreatedBy.Email
        FROM Task
        WHERE Id IN :Trigger.new
    ];

    // Initialize a map of qualyfing tasks, keyed on the Related To (WhatId)
    // field on the task. This will be used later to pull in Task.CreatedDate
    // when updating accounts

    Map<Id, Task> qualifyingTasksByWhatId = new Map<Id, Task>();

    // Go through the new tasks and check to see whether the task qualifies
    // based on the email address and subject criteria

    for (Task eachTask : newTasks) {
        if (eachTask.WhatId != null) {
            if (eachTask.CreatedBy.Email.contains('@mobilemiles.nl')
                && eachTask.Subject.contains('Made call to')) {

                // Put the qualifying task in the map

                qualifyingTasksByWhatId.put(eachTask.WhatId, eachTask);
            }
        }   // if (eachTask.WhatId != null)
    }   // for each Task in newTasks

    // Look for accounts that were affected, by querying the account object.
    // This avoids situations where the task is not related to an account but
    // to some other type of record in the system.

    List<Account> affectedAccounts = [
        SELECT Id
        FROM Account
        WHERE Id IN :qualifyingTasksByWhatId.keySet()
    ];

    // Update any affected accounts

    if (affectedAccounts.size() > 0) {
        for (Account eachAccount : affectedAccounts) {
            eachAccount.Laatste_activiteit_Mobile_Miles__c =
                qualifyingTasksByWhatId.get(eachAccount.Id).CreatedDate;
        }

        update affectedAccounts;
    }
}