def merge_full_elog_by_customer_and_date(entry):
    """Aggregate sales for each customer and date pair.

    Takes a collection of transactions grouped by customer_id and date and
    returns a single record with the sum of all those transaction values.

    Args:
        entry: Entry records from the previous pipeline step, grouped by
            customer_id and date.

    Returns:
        Tuple containing customer_id, date_str, date and the sum of all the
        sales in the grouped records.
    """
    (customer_id, date_str), records = entry
    sales = 0.0
    date = None
    for r in records:
        if not date:
            date = r[2]
        sales += r[3]
    return customer_id, date_str, date, round(sales, 2)