AccountTriggerHandler_snippet.cls

    // Snippet from the AccountTriggerHandler
    ...
        // If the mock data is not being used
        // creates new instances of the real deal
        public AccountTriggerHandler() {
            this(new DataLayer(), new FieldReader());
        }

        // Child_Accounts__r is a nonwritable child relationship
        // Uses the field reader to retrieve the list of child records:
        List<Account> childRecords =
            fieldReader.getFieldRecords(parentAndChildAccount, 'Child_Accounts__r');


        // Updates the field Latest Child URL with the value from External_URL__c
        // External_URL__c is a formula field (nonwritable string)
        newRecord.Latest_Child_URL__c = 
            (String)fieldReader.getFieldValue(latestChild, 'External_URL__c');