7

我有一个特定的计算字段，我经常希望在 Linq 查询的“选择”字段中返回，例如今年的客户订单总额，以及客户的其他人口统计信息。

public class Customer {
  public decimal TotalPurchasesThisYear(MyDataContext db) {
    return db.Orders.Where(o => o.CustomerID == ID)
                    .Sum(o => o.OrderTotalAmt);
  }
}

public class SomeReport {
  public void GetCustomerInfoBySalesperson(long salespersonID) {
    using (var db = new MyDataContext()) {
      var q = db.Customers.Where(c => c.SalespersonID == salespersonID)
                          .Select(c => new { c.Name, c.Address, ThisYearPurchases = c.TotalPurchasesThisYear(db) })
                          .ToList();
      // etc..
    }
  }
}

显然，这是行不通的，因为TotalPurchasesThisYear没有 SQL 翻译。但它里面的所有东西都有一个 SQL 翻译。我不想直接在查询中包含该代码，因为我在很多地方都在做同样的计算。我的直觉告诉我这应该用 an 来完成，Expression但我已经玩过并且不能完全找出正确的语法。

帮助，有人吗？谢谢！

4

2 回答 2

0

好的，我找到了一种方法来做到这一点，但我不确定这是最好的方法：

public class CustomerOrderInfo {
  public long CustomerID;
  public decimal TotalPurchases;
}

public class Customer {
  public static Expression<Func<Customer, CustomerOrderInfo>> GetCustomerOrderInfo() {
    return c => new CustomerOrderInfo {
                  CustomerID = c.ID,
                  TotalPurchases = c.Orders.Where(o => o.OrderDate.Year == DateTime.Now.Year)
                                    .Sum(o => o.OrderTotalAmt)
                };
  }
}

public class SomeReport {
  public void GetCustomerInfoBySalesperson(long salespersonID) {
    using (var db = new MyDataContext()) {
      var q = db.Customers
                .Join(db.Customers.Select(Customer.GetCustomerOrderInfo()),
                      c => c.ID, i => i.CustomerID,
                      (c, i) => new { c.Name, c.Address, i.TotalPurchases })
                .ToList();
  // etc..
    }
  }
}

它可以工作......但底层 SQL 最终以嵌套查询的连接结束，这不是我的理想。我希望看到底层 SQL 变得简单，例如：

select c.Name, c.Address, sum(o.OrderTotalAmt) TotalPurchases
from Customer c 
left join [Order] o on o.CustomerID = c.ID
where Year(o.OrderDate) = @year
group by c.Name, c.Address
于 2013-04-09T09:02:06.393 回答
0 
List<Parent> myParents = new List<Parent>
                                         {
                                             new Parent() 
                                             {
                                                 Prop1 = "1", 
                                                 Prop2 = "2",
                                                 Prop3 = "3", 
                                                 Children = new List<Child>()
                                                        {
                                                            new Child(){ Prop1 = 1, Prop2 = 2, Prop3 = 3 },
                                                            new Child(){ Prop1 = 21, Prop2 = 22, Prop3 = 23 },
                                                            new Child(){ Prop1 = 31, Prop2 = 32, Prop3 = 33 }
                                                        }
                                             }, 
                                         };

            Expression<Func<Parent, int>> GetChildSum =
                p => p.Children.Where(c => c.Prop1 > 0).Sum(o => o.Prop2);

            var v = myParents.Where(w => w.Prop1 == "1").Select(p => GetChildSum.Compile().Invoke(p)).ToList();

            Console.WriteLine(v.First());

            //output is 56
于 2013-04-18T13:24:57.957 回答