如何按字段对列表进行分组，并通过linq选择另一个字段的分组min？

Question

我有一个航班的列表，我试图用日期字段对它进行分组，然后选择它进入每个组的最低价格。在结果我应该得到 每天最便宜的航班。

(from flights in aFlightList 
            where flights.IsDeparture && flights.OnlyTour == "0" && flights.OnlyPhone == "0" 
            group flights by flights.FlightDateFull 
             into grouping 
             select grouping.OrderBy(a => a.FlightDateFull).First()).ToArray(); 

此代码按日期分组列表，但无法获得特价机票。 我试试这样：

(from flights in aFlightList 
            where flights.IsDeparture && flights.OnlyTour == "0" && flights.OnlyPhone == "0" 
            group flights by flights.FlightDateFull 
             into grouping 
             select grouping.OrderBy(a => a.FlightDateFull).Min(d=>d.PriceView)).ToArray(); 

这样就会出错。

如何做到这一点？

Answer 1

group by操作的结果是一个分组列表，其中分组由一个密钥和共享该密钥的元素列表组成。

让我们开始您的第一个查询。按操作分组按日期分组aFlightList，因此不需要按日期进一步排序组元素（它是一个且相同的）。但您可以按价格订购，因此First将以最低价格返回元素。最后，由于group by生成的组的顺序可能不是您想要的，您可以通过密钥（或其一部分）来订购分组。

与所有他这样说，修改后的查询可能是这样的：

(from flights in aFlightList 
where flights.IsDeparture && flights.OnlyTour == "0" && flights.OnlyPhone == "0" 
group flights by flights.FlightDateFull 
into grouping 
orderby grouping.Key 
select grouping.OrderBy(e => e.PriceView).First()) 
.ToArray(); 

Answer 2

您可以创建你想要的结果的新对象，并在执行选择。例如：

class Flight 
    { 
     public bool IsDeparture; 
     public string OnlyTour; 
     public string OnlyPhone; 
     public DateTime FlightDateFull; 
     public decimal PriceView; 
    } 

    [TestMethod] 
    public void FlightTest() 
    { 
     // Arrange 
     var flightList = new List<Flight> { 
      new Flight { IsDeparture = true, OnlyPhone = "0", OnlyTour = "0", FlightDateFull = new DateTime(2016,8,7), PriceView = 1 }, 
      new Flight { IsDeparture = true, OnlyPhone = "0", OnlyTour = "0", FlightDateFull = new DateTime(2016,8,7), PriceView = 2 }, 
      new Flight { IsDeparture = true, OnlyPhone = "0", OnlyTour = "0", FlightDateFull = new DateTime(2016,8,8), PriceView = 2 }, 
      new Flight { IsDeparture = true, OnlyPhone = "0", OnlyTour = "0", FlightDateFull = new DateTime(2016,8,8), PriceView = 3 } 
     }; 

     // Act 
     var result = (from flights in flightList 
         where flights.IsDeparture && flights.OnlyTour == "0" && flights.OnlyPhone == "0" 
         group flights by flights.FlightDateFull into grouping 
         select new { Date = grouping.Key, MinPrice = grouping.Min(a => a.PriceView) }).OrderBy(a => a.Date).ToList(); 

     // Assert 
     Assert.AreEqual(new DateTime(2016, 8, 7), result[0].Date); 
     Assert.AreEqual(1, result[0].MinPrice); 
     Assert.AreEqual(new DateTime(2016, 8, 8), result[1].Date); 
     Assert.AreEqual(2, result[1].MinPrice); 
    }