Question

假设我有Customer和Order个对象，其中一个Customer可以有多个Orders（因此Order类有一个CustomerId我希望返回所有CustomerAndMostRecentOrder个对象的集合，其定义如下：

public class CustomerAndMostRecentOrder
{
  public Customer Customer { get; set; }
  public Order MostRecentOrder { get; set; }
}

我如何编写执行此操作的Linq查询（我正在使用Linq to SQL）？

Answer 1

您可以使用以下查询：

from c in customers
select new CustomerAndMostRecentOrder
    {
        Customer = c,
        MostRecentOrder = c.Orders.OrderByDescending(o => o.PurchaseDate).FirstOrDefault()
    };

这将使用客户的导航属性进行订购。 MostRecentOrder是通过在某些DateTime属性上对Orders进行排序然后加载第一个来获得的。

Answer 2

您需要在CreatedDate表格中设置Order日期才能获得最新订单。然后，要获取CustomerAndMostRecentOrder对象，请执行以下查询：

from c in customers
join o in orders on c.ID equals o.CustomerID into co
select new CustomerAndMostRecentOrder
{
    Customer = c,
    MostRecentOrder = co.OrderByDescending(o => o.CreatedDate).FirstOrDefault()
}

Answer 3

public class CustomerAndMostRecentOrder
{
    public CustomerAndMostRecentOrder(Customer customer, Order mostRecentOrder)
    {
        Customer = customer;
        MostRecentOrder = mostRecentOrder;
    }

    public Customer Customer { get; set; }
    public Order MostRecentOrder { get; set; }
}

public class Order
{
}


public class Customer
{
    public IEnumerable<Order> GetOrders()
    {

    }
}


public static class UsageClass
{

    public static void Sample(IEnumerable<Customer> allCustomers)
    {
        IEnumerable<CustomerAndMostRecentOrder> customerAndMostRecentOrders =
            allCustomers.Select(customer => new CustomerAndMostRecentOrder(customer, customer.GetOrders().Last()));
    }

}

Answer 4

作为另一种选择，您可能需要查看http://msdn.microsoft.com/en-us/library/system.data.linq.dataloadoptions.associatewith.aspx中讨论的DataLoadOptions.AssociateWith。只需根据上下文设置您的要求，您就不必担心在查询级别过滤子项。

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