Entity Framework Tutorial Basics（36）：Eager Loading

追憶似水流年發表於2016-07-07
Framework

Eager Loading:

Eager loading is the process whereby a query for one type of entity also loads related entities as part of the query. Eager loading is achieved using the Include() method.

In the following example, it gets all the students from the database along with it's standards using Include() method.

LINQ Query Syntax:

using (var context = new SchoolDBEntities())
{
       var res = (from s in context.Students.Include("Standard")
                        where s.StudentName == "Student1"
                        select s).FirstOrDefault<Student>();
}

 

LINQ Method Syntax:

using (var ctx = new SchoolDBEntities())
{
       stud = ctx.Students.Include("Standard")
                               .Where(s => s.StudentName == "Student1").FirstOrDefault<Student>();
           
}

 

The code shown above will result in following SQL query:

SELECT TOP (1) 
[Extent1].[StudentID] AS [StudentID], 
[Extent1].[StudentName] AS [StudentName], 
[Extent2].[StandardId] AS [StandardId], 
[Extent2].[StandardName] AS [StandardName], 
[Extent2].[Description] AS [Description]
FROM  [dbo].[Student] AS [Extent1]
LEFT OUTER JOIN [dbo].[Standard] AS [Extent2] ON [Extent1].[StandardId] = [Extent2].[StandardId]
WHERE 'Student1' = [Extent1].[StudentName]

 

Use Lambda Expression:

You can also use linq lambda expression in Include method. For this, take a reference ofSystem.Data.Entity namespace and use lambda expression as shown below.

using System;
using System.Data.Entity; 
   
class Program
{
    static void Main(string[] args)
    {
    
        using (var ctx = new SchoolDBEntities())
        {
            stud = ctx.Students.Include(s => s.Standard)
                                .Where(s => s.StudentName == "Student1")
                                .FirstOrDefault<Student>();
           
        }
    }
}

 

The code shown above will result in the following SQL query:

SELECT TOP (1) 
[Extent1].[StudentID] AS [StudentID], 
[Extent1].[StudentName] AS [StudentName], 
[Extent2].[StandardId] AS [StandardId], 
[Extent2].[StandardName] AS [StandardName], 
[Extent2].[Description] AS [Description]
FROM  [dbo].[Student] AS [Extent1]
LEFT OUTER JOIN [dbo].[Standard] AS [Extent2] ON [Extent1].[StandardId] = [Extent2].[StandardId]
WHERE 'Student1' = [Extent1].[StudentName]

 

Load multiple levels of related entities:

You can also eagerly load multiple levels of related entities. The code snippet shown below loads related Student, Standard and Teachers:

using (var ctx = new SchoolDBEntities())
{
        stud = ctx.Students.Include("Standard.Teachers")
                            .Where(s => s.StudentName == "Student1")
                            .FirstOrDefault<Student>();
}

 

Or using lambda expression as below.

using (var ctx = new SchoolDBEntities())
{
        stud = ctx.Students.Include(s => s.Standard.Teachers)
                            .Where(s => s.StudentName == "Student1")
                            .FirstOrDefault<Student>();
}

 

The code shown above results in the following SQL query:

SELECT [Project2].[StudentID] AS [StudentID], 
[Project2].[StudentName] AS [StudentName], 
[Project2].[StandardId] AS [StandardId], 
[Project2].[StandardName] AS [StandardName], 
[Project2].[Description] AS [Description], 
[Project2].[C1] AS [C1], 
[Project2].[TeacherId] AS [TeacherId], 
[Project2].[TeacherName] AS [TeacherName], 
[Project2].[StandardId1] AS [StandardId1]
FROM ( SELECT 
    [Limit1].[StudentID] AS [StudentID], 
    [Limit1].[StudentName] AS [StudentName], 
    [Limit1].[StandardId1] AS [StandardId], 
    [Limit1].[StandardName] AS [StandardName], 
    [Limit1].[Description] AS [Description], 
    [Project1].[TeacherId] AS [TeacherId], 
    [Project1].[TeacherName] AS [TeacherName], 
    [Project1].[StandardId] AS [StandardId1], 
    CASE WHEN ([Project1].[TeacherId] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1]
    FROM   (SELECT TOP (1) [Extent1].[StudentID] AS [StudentID], [Extent1].[StudentName] AS [StudentName], [Extent1].[StandardId] AS [StandardId2], [Extent2].[StandardId] AS [StandardId1], [Extent2].[StandardName] AS [StandardName], [Extent2].[Description] AS [Description]
        FROM  [dbo].[Student] AS [Extent1]
        LEFT OUTER JOIN [dbo].[Standard] AS [Extent2] ON [Extent1].[StandardId] = [Extent2].[StandardId]
        WHERE 'updated student' = [Extent1].[StudentName] ) AS [Limit1]
    LEFT OUTER JOIN  (SELECT 
        [Extent3].[TeacherId] AS [TeacherId], 
        [Extent3].[TeacherName] AS [TeacherName], 
        [Extent3].[StandardId] AS [StandardId]
        FROM [dbo].[Teacher] AS [Extent3]
        WHERE [Extent3].[StandardId] IS NOT NULL ) AS [Project1] ON [Limit1].[StandardId2] = [Project1].[StandardId]
)  AS [Project2]
ORDER BY [Project2].[StudentID] ASC, [Project2].[StandardId] ASC, [Project2].[C1] ASC

 

Learn how Entity Framework supports lazy loading of entities, in the next section.

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