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📜  Entity Framework-渴望加载

📅  最后修改于: 2020-11-21 07:28:21             🧑  作者: Mango

急切加载是一种过程，通过该过程，对一种类型的实体的查询也将相关实体作为查询的一部分进行加载。急切的加载是通过使用Include方法实现的。

这意味着请求的相关数据将与数据库的查询结果一起返回。到数据源只有一个连接，在初始查询中返回了大量数据。

例如，在查询学生时，急于加载他们的注册。将在单个查询中检索学生及其注册。

让我们看下面的示例，在该示例中，使用急切加载从数据库中检索所有具有各自入学条件的学生。

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

以下是可以使用的其他一些急切的加载查询形式。

// 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

也可以急于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，以更好地理解。