LINQ - 嵌套查询

Question

我有一个SQL语句，我试图转换为LINQ查询。我需要这样做，因为我不能编辑我的数据库:(无论如何，我有一个SQL语句如下所示：

SELECT 
    CustomerID, 
    FirstName, 
    LastName, 
    Gender, 
    BirthMonth, 
    (SELECT COUNT(ID) FROM PurchaseOrder WHERE [CustomerID]=CustomerID) as TotalPurchases 
FROM 
    MyEntities 

我知道该怎么做的一切在LINQ除外的嵌套查询部分目前，我有以下几点：？

var results = from x in context.MyEntities 
       select new Customer() 
       { 
       CustomerID = x.CustomerID, 
       FirstName = x.FirstName, 
       LastName = x.LastName, 
       Gender = x.Gender, 
       BirthMonth = x.BirthMonth, 
       TotalPurchases = ? 
       }; 

如何执行在LINQ嵌套查询来获取TotalPurchases值

谢谢你这么多

Answer 1

！你可以只嵌套的LINQ to Entities查询，以及：

var results = from x in context.MyEntities 
       select new Customer() 
       { 
       CustomerID = x.CustomerID, 
       FirstName = x.FirstName, 
       LastName = x.LastName, 
       Gender = x.Gender, 
       BirthMonth = x.BirthMonth, 
       TotalPurchases = context.PurchaseOrders 
             .Where(po=>po.CustomerId == x.CustomerId) 
             .Count() 
       }; 

Answer 2

只是做一个报数“个PurchaseOrder”，我认为是对客户的实体的导航属性的。

TotalPurchases = x.PurchaseOrders.Count() 

Answer 3

通过BrokenGlass答案使用LINQ到实体时，并导致错误，指出“无法创建类型‘myEntity所’的恒定值，并没有为我工作。只有基本类型或枚举类型在这个支持语境“。

为了克服这一点，我使用了以下内容：

var results = from x in context.MyEntities 
       join po in context.PurchaseOrders on x.CustomerID equals po.CustomerID into purchaseOrders 
       select new Customer() 
       { 
       CustomerID = x.CustomerID, 
       FirstName = x.FirstName, 
       LastName = x.LastName, 
       Gender = x.Gender, 
       BirthMonth = x.BirthMonth, 
       TotalPurchases = purchaseOrders.Count() 
       };