实体框架 - 预加载

预加载是指查询一种类型的实体时，也会将相关实体作为查询的一部分进行加载的过程。预加载是通过使用 Include 方法 实现的。

这意味着请求相关数据与查询结果一起从数据库返回。与数据源只有一个连接，初始查询中会返回大量数据。

例如，查询学生时，预加载他们的注册信息。学生及其注册信息将在单个查询中检索。

让我们看下面的示例，其中使用预加载从数据库中检索所有学生及其各自的注册信息。

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

以下是一些可以使用的其他形式的预先加载查询。

// 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();

多级

也可以急切地加载多级相关实体。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。