使用 Linq 获取所有客户的最后订单日期

Question

我有以下两个POCO

public class Customers
{
    public Guid Id { get; set; }
    public string FirstName{ get; set; }
    public string LastName{ get; set; }
}

和

public class Orders
{
    public Guid Id { get; set; }
    public Guid CustomerId { get; set; }
    public string Product { get; set; }
    public DateTime Date { get; set; }
}

假设我的数据服务返回 List 和 List，我将如何使用 Linq 获取每个客户的最后一个订单的日期？

我想返回 Customer.Id、Customer.FirstName、Customer.LastName、Order.Id、Order.Date 和 Order.Product（作为控制器操作中的 Json），所以我还需要创建一个新的 POCO 来持有结果？类似于以下内容：

public class CustomerOrders
{
    public Guid CustomerId { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public Guid OrderId { get; set; }
    public DateTime Date { get; set; }
    public string Product { get; set; }
}

Answer 1

您可能希望像这样使用GROUP BY 和JOIN：

var customerOrders =
    (from c in customers
     join o in (from o1 in orders
                group o1 by o1.CustomerId into og
                select new { CustomerId = og.Key, Latest = og.Max(d => d.Date) })
     on c.Id equals o.CustomerId
     join o2 in orders
     on new {CustmerId = o.CustomerId, Date = o.Latest } 
     equals new { CustmerId = o2.CustomerId, Date = o2.Date }

     select new CustomerOrders
     {
         CustomerId = c.Id,
         FirstName = c.FirstName,
         LastName = c.LastName,
         OrderId = o2.Id,
         Date = o.Latest,
         Product = o2.Product
     });

如果您想要没有任何订单的客户的结果，请使用LEFT OUTER JOIN 和使用DefaultIfEmpty，如下所示：

var customerOrders =
    (from c in customers
     join o in (from o1 in orders
                group o1 by o1.CustomerId into og
                select new { CustomerId = og.Key, Latest = og.Max(d => d.Date) })
     on c.Id equals o.CustomerId into co
     from o in co.DefaultIfEmpty()  // LEFT OUTER JOIN 1
     join o2 in orders
     on new { CustmerId = (o == null)? Guid.Empty : o.CustomerId, Date = (o == null)? DateTime.MinValue : o.Latest }
     equals new { CustmerId = o2.CustomerId, Date = o2.Date } into oo
     from o2 in oo.DefaultIfEmpty()  // LEFT OUTER JOIN 2

     select new CustomerOrders
     {
         CustomerId = c.Id,
         FirstName = c.FirstName,
         LastName = c.LastName,
         OrderId = (o2 == null)? Guid.Empty : o2.Id,
         Date = (o == null)? DateTime.MinValue : o.Latest,
         Product = (o == null)? null : o2.Product
     });

请注意LEFT OUTER JOIN 可能会加入空值，因此您必须在 linq 查询中添加空检查。