实体框架 - 预加载

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预加载是对一种类型的实体的查询同时加载相关实体作为查询的一部分的过程。预加载是通过使用Include 方法来实现的。

这意味着请求相关数据与数据库的查询结果一起返回。仅与数据源建立一个连接，在初始查询中返回大量数据。

例如，在查询学生时，立即加载他们的注册情况。学生及其注册情况将在单个查询中检索。

让我们看一下下面的示例，其中使用预先加载从数据库中检索具有各自注册信息的所有学生。

class Program {

   static void Main(string[] args) {

      using (var context = new UniContextEntities()) {
         // Load all students and related enrollments
         var students = context.Students
            .Include(s ⇒ s.Enrollments).ToList();
			
         foreach (var student in students) {
            string name = student.FirstMidName + " " + student.LastName;
            Console.WriteLine("ID: {0}, Name: {1}", student.ID, name);
				
            foreach (var enrollment in student.Enrollments) {
               Console.WriteLine("Enrollment ID: {0}, Course ID: {1}", 
                  enrollment.EnrollmentID, enrollment.CourseID);
            }
         }

         Console.ReadKey();
      }
   }
}

编译并执行上述代码后，您将收到以下输出。

ID: 1, Name: Ali Alexander
       Enrollment ID: 1, Course ID: 1050
       Enrollment ID: 2, Course ID: 4022
       Enrollment ID: 3, Course ID: 4041
ID: 2, Name: Meredith Alonso
       Enrollment ID: 4, Course ID: 1045
       Enrollment ID: 5, Course ID: 3141
       Enrollment ID: 6, Course ID: 2021
ID: 3, Name: Arturo Anand
       Enrollment ID: 7, Course ID: 1050
ID: 4, Name: Gytis Barzdukas
       Enrollment ID: 8, Course ID: 1050
       Enrollment ID: 9, Course ID: 4022

以下是可以使用的一些其他形式的预加载查询。

// Load one Student and its related enrollments

var student1 = context.Students
   .Where(s ⇒ s.FirstMidName == "Ali")
   .Include(s ⇒ s.Enrollments).FirstOrDefault();

// Load all Students and related enrollments
// using a string to specify the relationship

var studentList = context.Students
   .Include("Enrollments").ToList();

// Load one Student and its related enrollments
// using a string to specify the relationship

var student = context.Students
   .Where(s ⇒ s.FirstMidName == "Salman")
   .Include("Enrollments").FirstOrDefault();

多层次

还可以急切地加载多个级别的相关实体。以下查询显示了学生、注册和课程的示例。

// Load all Students, all related enrollments, and all related courses

var studentList = context.Students
   .Include(s ⇒ s.Enrollments.Select(c ⇒ c.Course)).ToList();

// Load all Students, all related enrollments, and all related courses
// using a string to specify the relationships

var students = context.Students
   .Include("Enrollments.Course").ToList();

我们建议您逐步执行上述示例，以便更好地理解。