實體框架 - 渴望載入

渴望載入是指查詢一種型別的實體時，也載入相關實體作為查詢的一部分的過程。渴望載入是透過使用Include 方法實現的。

這意味著請求將相關資料與資料庫查詢結果一起返回。只對資料來源進行一次連線，並在初始查詢中返回大量資料。

例如，在查詢學生時，渴望載入他們的註冊資訊。學生及其註冊資訊將在單個查詢中檢索。

讓我們看下面的示例，其中所有學生及其各自的註冊資訊都透過渴望載入從資料庫中檢索。

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();
      }
   }
}

當編譯並執行上述程式碼時，您將收到以下輸出。

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

以下是可使用的其他一些渴望載入查詢形式。

// 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();

多級

還可以渴望載入多級相關實體。以下查詢顯示了學生、註冊資訊和課程的示例。

// 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();

我們建議您逐步執行上述示例，以便更好地理解。

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