Object Query of EF

Database :

有三种方式来进行EF查询：LINQ to Entities, Entity SQL,和 Query Builder

1.获取所有的Racer（Query Builder）：

using (Formula1Entities data = new Formula1Entities(connection))
            {
                ObjectSet<Racer> racers = data.Racers;
                Console.WriteLine(racers.CommandText);
                Console.WriteLine(racers.ToTraceString());
            }

racers.CommandText：[Formula1Entities].[Racers]

racers.ToTraceString()：

SELECT [Extent1].[Id] AS [Id],[Extent1].[Firstname] AS [Firstname],[Extent1].[Lastname] AS [Lastname],[Extent1].[Country] AS [Country],[Extent1].[Starts] AS [Starts],[Extent1].[Wins] AS [Wins]
FROM [dbo].[Racers] AS [Extent1]

另一种方式（和直接通过访问属性的方式类似）：

ObjectQuery<Racer> racers = data.CreateQuery<Racer>("[Formula1Entities].[Racers]");

2.带条件的查询（Query Builder）：

using (Formula1Entities data = new Formula1Entities( ))
           {
               string country = "Brazil";
               ObjectQuery<Racer> racers = data.Racers.Where("it.Country = @Country", new ObjectParameter("Country", country));
           }

public ObjectQuery<T> Where(string predicate, params ObjectParameter[] parameters)：定义在ObjectQuery中。

Where("it.Country = @Country", new ObjectParameter("Country", country))中的it表示集合中的一个item，后面紧跟着参数，需要注意的是ObjectParameter中的Country前面没有@符号

同理，也是可以通过CreateQuery方法来进行带条件的查询的：

using (Formula1Entities data = new Formula1Entities( ))
          {
              string country = "Brazil";
              ObjectQuery<Racer> racers = data.CreateQuery<Racer>("SELECT VALUE it FROM ([Formula1Entities].[Racers]) AS it WHERE it.Country = @Country",
              new ObjectParameter("Country", country));
          }

3.选择来自美国的Racer的前三个（Query Builder）：

using (var data = new Formula1Entities())
            {
                string country = "USA";
                ObjectQuery<Racer> racers = data.Racers.Where("it.Country = @Country",
                new ObjectParameter("Country", country))
                .OrderBy("it.Wins DESC, it.Starts DESC")
                .Top("3");
                foreach (var racer in racers)
                {
                    Console.WriteLine("{0} {1}, wins: {2}, starts: {3}",
                    racer.Firstname, racer.Lastname, racer.Wins, racer.Starts);
                }
            }