private void productSelectComboBox_SelectedIndexChanged(object sender, System.EventArgs e)
        {
            // get the selected product
            if (productSelectComboBox.SelectedItem == null)
            {
                return;
            }

            ProductEntity selectedProduct = (ProductEntity)productSelectComboBox.SelectedItem;

            _productFilter.Value = selectedProduct.ProductId;

            // select the customers
            CustomerCollection customers = new CustomerCollection();

            // select the customers using the relations and the filter we defined
            customers.GetMulti(_productFilter, 0, _customerSorter, _relationsToWalk);

            // bind them
            customersDataGrid.DataSource = customers;

            // get scalar for amount of times this product is sold.
            OrderDetailCollection orderDetails = new OrderDetailCollection();
            object scalarValue = orderDetails.GetScalar(
                OrderDetailFieldIndex.Quantity, null, AggregateFunction.Sum, (OrderDetailFields.ProductId == selectedProduct.ProductId));
            int amountUnitSold = 0;

            if (scalarValue != System.DBNull.Value)
            {
                amountUnitSold = (int)scalarValue;
            }
            _amountUnitsSoldTextBox.Text = amountUnitSold.ToString();
        }