Entity framework 加载多层相关实体数据

Entity framework有3种加载数据的方式：懒汉式（Lazy loading），饿汉式（Eager loading），显示加载（Explicit loading）。3种加载方式有各自的优缺点，详细的可以参考微软的技术文章《Reading Related Data with the Entity Framework in an ASP.NET MVC Application》

本文只针对需要一次性加载多个相关联实体的情况进行讨论。

 

1. 数据库模型

 

 

2. Model和DbContext

 

 public class Student
     {
         public int ID { get; set; }

         public string LastName { get; set; }

         public string FirstMidName { get; set; }

         public DateTime? EnrollmentDate { get; set; }

         public virtual ICollection<Enrollment> Enrollments { get; set; }
     }

Student.cs

 public class Enrollment
     {
         public int EnrollmentID { get; set; }

         public int CourseID { get; set; }

         public int StudentID { get; set; }

         public int? Grade { get; set; }

         public virtual Student Student { get; set; }

         public virtual Course Course { get; set; }
     }

Enrollment.cs

 public class Course
     {
         public int CourseID { get; set; }

         public string Title { get; set; }

         public string Credits { get; set; }

         public virtual ICollection<Enrollment> Enrollments { get; set; }
     }

Course

 public class Context:DbContext
     {
         public Context() : base("ReadMultipleEntity")
         {

         }

         public DbSet<Student> Students { get; set; }

         public DbSet<Course> Courses { get; set; }

         public DbSet<Enrollment> Enrollments { get; set; }

         protected override void OnModelCreating(DbModelBuilder modelBuilder)
         {
             modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();
         }

     }

Context.cs

 

3. 需求

 

想要得到所有学生及其选课名称的列表

因为是想要所有学生及课程的列表信息，所以此时选择饿汉式效率最佳（一次性将所有数据加载到内存）。

context.Students.Include(x => x.Enrollments).ToList();//加载Students和Enrollments实体

 

通过Sql Profiler监控到所执行的SQL

 SELECT
     [Project1].[ID] AS [ID],
     [Project1].[LastName] AS [LastName],
     [Project1].[FirstMidName] AS [FirstMidName],
     [Project1].[EnrollmentDate] AS [EnrollmentDate],
     [Project1].[C1] AS [C1],
     [Project1].[EnrollmentID] AS [EnrollmentID],
     [Project1].[CourseID] AS [CourseID],
     [Project1].[StudentID] AS [StudentID],
     [Project1].[Grade] AS [Grade]
     FROM ( SELECT
         [Extent1].[ID] AS [ID],
         [Extent1].[LastName] AS [LastName],
         [Extent1].[FirstMidName] AS [FirstMidName],
         [Extent1].[EnrollmentDate] AS [EnrollmentDate],
         [Extent2].[EnrollmentID] AS [EnrollmentID],
         [Extent2].[CourseID] AS [CourseID],
         [Extent2].[StudentID] AS [StudentID],
         [Extent2].[Grade] AS [Grade],
         CASE WHEN ([Extent2].[EnrollmentID] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1]
         FROM  [dbo].[Student] AS [Extent1]
         LEFT OUTER JOIN [dbo].[Enrollment] AS [Extent2] ON [Extent1].[ID] = [Extent2].[StudentID]
     )  AS [Project1]
     ORDER BY [Project1].[ID] ASC, [Project1].[C1] ASC

Only Enrollment

如果想将和Enrollment相关联的Course实体同时加载，则需要如下语句：

context.Students.Include(x => x.Enrollments.Select(y => y.Course)).ToList();

 

通过Sql Profiler监控到所执行的SQL

 SELECT
     [Project1].[ID] AS [ID],
     [Project1].[LastName] AS [LastName],
     [Project1].[FirstMidName] AS [FirstMidName],
     [Project1].[EnrollmentDate] AS [EnrollmentDate],
     [Project1].[C1] AS [C1],
     [Project1].[EnrollmentID] AS [EnrollmentID],
     [Project1].[CourseID] AS [CourseID],
     [Project1].[StudentID] AS [StudentID],
     [Project1].[Grade] AS [Grade],
     [Project1].[CourseID1] AS [CourseID1],
     [Project1].[Title] AS [Title],
     [Project1].[Credits] AS [Credits]
     FROM ( SELECT
         [Extent1].[ID] AS [ID],
         [Extent1].[LastName] AS [LastName],
         [Extent1].[FirstMidName] AS [FirstMidName],
         [Extent1].[EnrollmentDate] AS [EnrollmentDate],
         [Join1].[EnrollmentID] AS [EnrollmentID],
         [Join1].[CourseID1] AS [CourseID],
         [Join1].[StudentID] AS [StudentID],
         [Join1].[Grade] AS [Grade],
         [Join1].[CourseID2] AS [CourseID1],
         [Join1].[Title] AS [Title],
         [Join1].[Credits] AS [Credits],
         CASE WHEN ([Join1].[EnrollmentID] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1]
         FROM  [dbo].[Student] AS [Extent1]
         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]
             FROM  [dbo].[Enrollment] AS [Extent2]
             INNER JOIN [dbo].[Course] AS [Extent3] ON [Extent2].[CourseID] = [Extent3].[CourseID] ) AS [Join1] ON [Extent1].[ID] = [Join1].[StudentID]
     )  AS [Project1]
     ORDER BY [Project1].[ID] ASC, [Project1].[C1] ASC

Enrollment and Course

比较两次SQL可得，后者多join了一个Course表，从而一次性得到所有的结果。如果想在Course再连接其他实体，可以仿照上面的形式。

 

源码参考