Dapper多对象查询一对多

public class Order
{
    public int OrderId { get; set; }
    public string OrderNumber { get; set; }
    // 其他属性...
    public List<OrderItem> OrderItems { get; set; }
}

public class OrderItem
{
    public int OrderItemId { get; set; }
    public int OrderId { get; set; }
    public string ProductName { get; set; }
    // 其他属性...
}

public List<Order> GetOrdersWithItems()
{
    using (var connection = new SqlConnection(connectionString))
    {
        string query = "SELECT o.OrderId, o.OrderNumber, oi.OrderItemId, oi.ProductName " +
                       "FROM Orders o " +
                       "JOIN OrderItems oi ON o.OrderId = oi.OrderId";

        var ordersDictionary = new Dictionary<int, Order>();

        connection.Query<Order, OrderItem, Order>(query, (order, orderItem) =>
        {
            Order existingOrder;
            if (!ordersDictionary.TryGetValue(order.OrderId, out existingOrder))
            {
                existingOrder = order;
                existingOrder.OrderItems = new List<OrderItem>();
                ordersDictionary.Add(existingOrder.OrderId, existingOrder);
            }

            existingOrder.OrderItems.Add(orderItem);
            return existingOrder;
        }, splitOn: "OrderItemId");

        return ordersDictionary.Values.ToList();
    }
}