《Entity Framework 6 Recipes》中文翻译系列 (18) -----第三章查询之结果集扁平化和多属性分组

翻译的初衷以及为什么选择《Entity Framework 6 Recipes》来学习，请看本系列开篇

3-14 结果集扁平化

问题

　　你有一对多关联的两个实体，你想通过一个查询，获取关联中的两个实体的扁平化投影。扁平化或者叫压缩，这是不规范的叫法。它是指一个有父类和子类的对象图，被投影到一个单独的类中。

解决方案

　　假设你有一对拥有一对多关联的实体，如图3-15所示的模型。

图3-15 模型中，一个代表助理的Associate的实体类型和一个代表助理工资历史的AssociateSalary实体

　　你想在一个查询中获取所有的associates 和他们的工资历史，可能有些新员工在系统还没有工资记录。你希望查询结果集中能包含这些关联。

　　请按代码清单3-30的方式，查询模型并获取你想要的结果集。

代码清单3-30. 使用LINQ和Entity SQL扁平化结果集

 using (var context = new EFRecipesEntities())

             {

                 // 删除之前的测试数据

                 context.Database.ExecuteSqlCommand("delete from chapter3.associatesalary");

                 context.Database.ExecuteSqlCommand("delete from chapter3.associate");

                 // 添加新的测试数据

                 var assoc1 = new Associate { Name = "Janis Roberts" };

                 var assoc2 = new Associate { Name = "Kevin Hodges" };

                 var assoc3 = new Associate { Name = "Bill Jordan" };

                 var salary1 = new AssociateSalary

                 {

                     Salary = 39500M,

                     SalaryDate = DateTime.Parse("8/4/09")

                 };

                 var salary2 = new AssociateSalary

                 {

                     Salary = 41900M,

                     SalaryDate = DateTime.Parse("2/5/10")

                 };

                 var salary3 = new AssociateSalary

                 {

                     Salary = 33500M,

                     SalaryDate = DateTime.Parse("10/08/09")

                 };

                 assoc2.AssociateSalaries.Add(salary1);

                 assoc2.AssociateSalaries.Add(salary2);

                 assoc3.AssociateSalaries.Add(salary3);

                 context.Associates.Add(assoc1);

                 context.Associates.Add(assoc2);

                 context.Associates.Add(assoc3);

                 context.SaveChanges();

             }

             using (var context = new EFRecipesEntities())

             {

                 Console.WriteLine("Using LINQ...");

                 var allHistory = from a in context.Associates

                                  from ah in a.AssociateSalaries.DefaultIfEmpty()

                                  orderby a.Name

                                  select new

                                  {

                                      Name = a.Name,

                                      Salary = (decimal?)ah.Salary,

                                      Date = (DateTime?)ah.SalaryDate

                                  };

                 Console.WriteLine("Associate Salary History");

                 foreach (var history in allHistory)

                 {

                     if (history.Salary.HasValue)

                         Console.WriteLine("{0} Salary on {1} was {2}", history.Name,

                                            history.Date.Value.ToShortDateString(),

                                            history.Salary.Value.ToString("C"));

                     else

                         Console.WriteLine("{0} --", history.Name);

                 }

             }

             using (var context = new EFRecipesEntities())

             {

                 Console.WriteLine("\nUsing Entity SQL...");

                 var esql = @"select a.Name, h.Salary, h.SalaryDate

                  from Associates as a outer apply

                    a.AssociateSalaries as h order by a.Name";

                 var allHistory = ((IObjectContextAdapter)context).ObjectContext.CreateQuery<DbDataRecord>(esql);

                 Console.WriteLine("Associate Salary History");

                 foreach (var history in allHistory)

                 {

                     if (history["Salary"] != DBNull.Value)

                         Console.WriteLine("{0} Salary on {1:d} was {2:c}", history["Name"],

                                            history["SalaryDate"], history["Salary"]);

                     else

                         Console.WriteLine("{0} --", history["Name"]);

                 }

             }

             Console.WriteLine("\nPress <enter> to continue...");

             Console.ReadLine();

         }

　　这里的诀窍是，我们“扁平化”（flatten）层次结构的数据，比如，一个associate和多个 salary输入。代码清单3-30输出如下：

 Using LINQ...

 Associate Salary History

 Bill Jordan Salary on // was $,500.00

 Janis Roberts --Kevin Hodges Salary on // was $,500.00

 Kevin Hodges Salary on // was $,900.00

 Using Entity SQL...

 Bill Jordan Salary on // was $,500.00

 Janis Roberts --Kevin Hodges Salary on // was $,500.00

 Kevin Hodges Salary on // was $,900.00

原理

　　为了扁平化结果集，我们使用了3-10中的策略、使用嵌套形式的from从句和DefaultIfEmpty()方法来获得两张表的一个左外连接。方法DefaultIfEmpty()能确保我们有左边表(Associate 实体）的所有行，即使右边（AssociateSalary实体）没有与它对应的行。我们将结果集投影到一个匿名类型，当没有AssociateSalary实体与Associate实体对应时，小心属性salary 和 salarydate得到null值。

　　对于Entity SQL 解决方案，我们使用 outer apply 操作符创建Associate实体和AssociateSalary实体之间的匹配。在SQL Server中,可以使用corss和outer apply操作符。

3-15 使用多属性分组

问题

　　你想在一个查询中使用多属性对结果集进行分组，以致在数据库中执行查询时使用多列进行分组。

解决方案

　　假设你有包含一个Enent实体的模型，如图3-16所示。Event有属性name,city和sate。你想通过state和city对events进行分组。

图3-16 包含一个Enent实体的模型，Event有属性name,city和sate属性

　　在代码清单3-31中，我们使用Code-First方法创建了实体类。

代码清单3-31.实体类型

 public class Event

     {

         public int EventId { get; set; }

         public string Name { get; set; }

         public string State { get; set; }

         public string City { get; set; }

     }

　　接下来，代码清单3-32中创建了上下文对象，它是用Code-First方法访问实体框架功能的入口。

代码清单3-32. 上下文对象

  public class EFRecipesEntities : DbContext

     {

         public EFRecipesEntities()

             : base("ConnectionString") {}

         public DbSet<Event> Events { get; set; }

         protected override void OnModelCreating(DbModelBuilder modelBuilder)

         {

             modelBuilder.Entity<Event>().ToTable("Chapter3.Event");

             base.OnModelCreating(modelBuilder);

         }

     }

　　使用代码清单3-33中的代码，获取所有的events并按sate和city对结果集分组

代码清单3-33 多属性分组

  using (var context = new EFRecipesEntities())

             {

                 // 删除之前的测试数据

                 context.Database.ExecuteSqlCommand("delete from chapter3.event");

                 //添加新的测试数据

                 context.Events.Add(new Event

                 {

                     Name = "TechFest 2010",

                     State = "TX",

                     City = "Dallas"

                 });

                 context.Events.Add(new Event

                 {

                     Name = "Little Blue River Festival",

                     State = "MO",

                     City = "Raytown"

                 });

                 context.Events.Add(new Event

                 {

                     Name = "Fourth of July Fireworks",

                     State = "MO",

                     City = "Raytown"

                 });

                 context.Events.Add(new Event

                 {

                     Name = "BBQ Ribs Championship",

                     State = "TX",

                     City = "Dallas"

                 });

                 context.Events.Add(new Event

                 {

                     Name = "Thunder on the Ohio",

                     State = "KY",

                     City = "Louisville"

                 });

                 context.SaveChanges();

             }

             using (var context = new EFRecipesEntities())

             {

                 Console.WriteLine("Using LINQ");

                 var results = from e in context.Events

                               // 使用匿名类型封闭复合key State 和City

                               group e by new { e.State, e.City } into g

                               select new

                               {

                                   State = g.Key.State,

                                   City = g.Key.City,

                                   Events = g

                               };

                 Console.WriteLine("Events by State and City...");

                 foreach (var item in results)

                 {

                     Console.WriteLine("{0}, {1}", item.City, item.State);

                     foreach (var ev in item.Events)

                     {

                         Console.WriteLine("\t{0}", ev.Name);

                     }

                 }

             }

             using (var context = new EFRecipesEntities())

             {

                 Console.WriteLine("\nUsing Entity SQL");

                 var esql = @"select e.State, e.City, GroupPartition(e) as Events

                  from Events as e

                  group by e.State, e.City";

                 var records = ((IObjectContextAdapter)context).ObjectContext.CreateQuery<DbDataRecord>(esql);

                 Console.WriteLine("Events by State and City...");

                 foreach (var rec in records)

                 {

                     Console.WriteLine("{0}, {1}", rec["City"], rec["State"]);

                     var events = (List<Event>)rec["Events"];

                     foreach (var ev in events)

                     {

                         Console.WriteLine("\t{0}", ev.Name);

                     }

                 }

             }

             Console.WriteLine("\nPress <enter> to continue...");

             Console.ReadLine();

代码清单3-33的输出如下：

Using LINQ

Events by State and City...

Louisville, KY

Thunder on the Ohio

Raytown, MO

Little Blue River Festival

Fourth of July Fireworks

Dallas, TX

TechFest

BBQ Ribs Championship

Using Entity SQL

Events by State and City...

Louisville, KY

Thunder on the Ohio

Raytown, MO

Little Blue River Festival

Fourth of July Fireworks

Dallas, TX

TechFest

BBQ Ribs Championship

原理

　　　在代码清单3-33中，针对这个问题，展示了两种不同的方法。　第一种方法使用LINQ和group by 操作符按sate和city对结果集进行分组。当用group by进行多属性分组时，我们创建了一个匿名类型对数据进行分组。使用into从句将分组放到g中，它是我们存放查询结果集的第二个序列。

　　我们把结果集从g中投影到第二个匿名类型中，通过从分组key的字段State（第一个匿名类型中）获取State值，从分组key的字段City中获取 City值，对于events 我只是简单地把分组的全部成员分配给它。

　　对于Entity SQL方法，我们只能投影group by 从句使用的列、常量或者从聚合函数计算得到的值。在我们示例中，我们投影state、city和分组中的events.

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