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Entity framework有3種加載數據的方式：懶漢式（Lazy loading），餓漢式（Eager loading），顯示加載（Explicit loading）。3種加載方式有各自的優缺點，詳細的可以參考微軟的技術文章《Reading Related Data with the Entity Framework in an ASP.NET MVC Application》 本文只針對需要一次性加載多個相關聯實體的情況進行討論。 1. 數據庫模型 2. Model和DbContext

 1 public class Student
 2     {
 3         public
 
 int ID { get; set; }
 4 
 5         public string LastName { get; set; }
 6 
 7         public string FirstMidName { get; set; }
 8 
 9         public DateTime? EnrollmentDate { get; set; }
10 
11         public virtual ICollection<Enrollment> Enrollments { get; set; }
12     }

Student.cs

 1
 
 public class Enrollment
 2     {
 3         public int EnrollmentID { get; set; }
 4 
 5         public int CourseID { get; set; }
 6 
 7         public int StudentID { get; set; }
 8 
 9         public int? Grade { get; set; }
10 
11         public virtual Student Student { get; set; }
12 
13         public
 
 virtual Course Course { get; set; }
14     }

Enrollment.cs

 1 public class Course
 2     {
 3         public int CourseID { get; set; }
 4 
 5         public string Title { get; set; }
 6 
 7         public string Credits { get; set; }
 8 
 9         public virtual ICollection<Enrollment> Enrollments { get; set; }
10     }

Course

 1 public class Context:DbContext
 2     {
 3         public Context() : base("ReadMultipleEntity")
 4         {
 5 
 6         }
 7 
 8         public DbSet<Student> Students { get; set; }
 9 
10         public DbSet<Course> Courses { get; set; }
11 
12         public DbSet<Enrollment> Enrollments { get; set; }
13 
14         protected override void OnModelCreating(DbModelBuilder modelBuilder)
15         {
16             modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();
17         }
18 
19     }

Context.cs 3. 需求 想要得到所有學生及其選課名稱的列表 因為是想要所有學生及課程的列表信息，所以此時選擇餓漢式效率最佳（一次性將所有數據加載到內存）。 context.Students.Include(x => x.Enrollments).ToList();//加載Students和Enrollments實體 通過Sql Profiler監控到所執行的SQL

 1 SELECT 
 2     [Project1].[ID] AS [ID], 
 3     [Project1].[LastName] AS [LastName], 
 4     [Project1].[FirstMidName] AS [FirstMidName], 
 5     [Project1].[EnrollmentDate] AS [EnrollmentDate], 
 6     [Project1].[C1] AS [C1], 
 7     [Project1].[EnrollmentID] AS [EnrollmentID], 
 8     [Project1].[CourseID] AS [CourseID], 
 9     [Project1].[StudentID] AS [StudentID], 
10     [Project1].[Grade] AS [Grade]
11     FROM ( SELECT 
12         [Extent1].[ID] AS [ID], 
13         [Extent1].[LastName] AS [LastName], 
14         [Extent1].[FirstMidName] AS [FirstMidName], 
15         [Extent1].[EnrollmentDate] AS [EnrollmentDate], 
16         [Extent2].[EnrollmentID] AS [EnrollmentID], 
17         [Extent2].[CourseID] AS [CourseID], 
18         [Extent2].[StudentID] AS [StudentID], 
19         [Extent2].[Grade] AS [Grade], 
20         CASE WHEN ([Extent2].[EnrollmentID] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1]
21         FROM  [dbo].[Student] AS [Extent1]
22         LEFT OUTER JOIN [dbo].[Enrollment] AS [Extent2] ON [Extent1].[ID] = [Extent2].[StudentID]
23     )  AS [Project1]
24     ORDER BY [Project1].[ID] ASC, [Project1].[C1] ASC

Only Enrollment

如果想將和Enrollment相關聯的Course實體同時加載，則需要如下語句： context.Students.Include(x => x.Enrollments.Select(y => y.Course)).ToList(); 通過Sql Profiler監控到所執行的SQL

 1 SELECT 
 2     [Project1].[ID] AS [ID], 
 3     [Project1].[LastName] AS [LastName], 
 4     [Project1].[FirstMidName] AS [FirstMidName], 
 5     [Project1].[EnrollmentDate] AS [EnrollmentDate], 
 6     [Project1].[C1] AS [C1], 
 7     [Project1].[EnrollmentID] AS [EnrollmentID], 
 8     [Project1].[CourseID] AS [CourseID], 
 9     [Project1].[StudentID] AS [StudentID], 
10     [Project1].[Grade] AS [Grade], 
11     [Project1].[CourseID1] AS [CourseID1], 
12     [Project1].[Title] AS [Title], 
13     [Project1].[Credits] AS [Credits]
14     FROM ( SELECT 
15         [Extent1].[ID] AS [ID], 
16         [Extent1].[LastName] AS [LastName], 
17         [Extent1].[FirstMidName] AS [FirstMidName], 
18         [Extent1].[EnrollmentDate] AS [EnrollmentDate], 
19         [Join1].[EnrollmentID] AS [EnrollmentID], 
20         [Join1].[CourseID1] AS [CourseID], 
21         [Join1].[StudentID] AS [StudentID], 
22         [Join1].[Grade] AS [Grade], 
23         [Join1].[CourseID2] AS [CourseID1], 
24         [Join1].[Title] AS [Title], 
25         [Join1].[Credits] AS [Credits], 
26         CASE WHEN ([Join1].[EnrollmentID] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1]
27         FROM  [dbo].[Student] AS [Extent1]
28         LEFT OUTER JOIN  (SELECT [Extent2].[EnrollmentID] AS [EnrollmentID], [Extent2].[CourseID] AS [CourseID1], [Extent2].[StudentID] AS [StudentID], [Extent2].[Grade] AS [Grade], [Extent3].[CourseID] AS [CourseID2], [Extent3].[Title] AS [Title], [Extent3].[Credits] AS [Credits]
29             FROM  [dbo].[Enrollment] AS [Extent2]
30             INNER JOIN [dbo].[Course] AS [Extent3] ON [Extent2].[CourseID] = [Extent3].[CourseID] ) AS [Join1] ON [Extent1].[ID] = [Join1].[StudentID]
31     )  AS [Project1]
32     ORDER BY [Project1].[ID] ASC, [Project1].[C1] ASC

Enrollment and Course

比較兩次SQL可得，後者多join了一個Course表，從而一次性得到所有的結果。如果想在Course再連接其他實體，可以仿照上面的形式。 源碼參考

Entity framework 加載多層相關實體數據