EF加载实体的方式

原文：Loading Related Entities

EF加载数据的方式：

1. 预加载 eager loading
2. 延迟加载 lazy loading
3. 显示加载 explicit loading

预先加载会加载所有相关的实体，通过Include方法来实现

 1 using (var context = new BloggingContext()) 

 2 { 

 3     // Load all blogs and related posts 

 4     var blogs1 = context.Blogs 

 5                           .Include(b => b.Posts) 

 6                           .ToList(); 

 7  

 8     // Load one blogs and its related posts 

 9     var blog1 = context.Blogs 

10                         .Where(b => b.Name == "ADO.NET Blog") 

11                         .Include(b => b.Posts) 

12                         .FirstOrDefault(); 

13  

14     // Load all blogs and related posts  

15     // using a string to specify the relationship 

16     var blogs2 = context.Blogs 

17                           .Include("Posts") 

18                           .ToList(); 

19  

20     // Load one blog and its related posts  

21     // using a string to specify the relationship 

22     var blog2 = context.Blogs 

23                         .Where(b => b.Name == "ADO.NET Blog") 

24                         .Include("Posts") 

25                         .FirstOrDefault(); 

26 }

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预先加载也支持多层级加载相关的实体

 1 using (var context = new BloggingContext()) 

 2 { 

 3     // Load all blogs, all related posts, and all related comments 

 4     var blogs1 = context.Blogs 

 5                        .Include(b => b.Posts.Select(p => p.Comments)) 

 6                        .ToList(); 

 7  

 8     // Load all users their related profiles, and related avatar 

 9     var users1 = context.Users 

10                         .Include(u => u.Profile.Avatar) 

11                         .ToList(); 

12  

13     // Load all blogs, all related posts, and all related comments  

14     // using a string to specify the relationships 

15     var blogs2 = context.Blogs 

16                        .Include("Posts.Comments") 

17                        .ToList(); 

18  

19     // Load all users their related profiles, and related avatar  

20     // using a string to specify the relationships 

21     var users2 = context.Users 

22                         .Include("Profile.Avatar") 

23                         .ToList(); 

24 }

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延迟加载只有相关的属性被访问时才去加载，延迟加载的的实现是在导航属性前加上virtual

1 public class Blog 

2 {  

3     public int BlogId { get; set; }  

4     public string Name { get; set; }  

5     public string Url { get; set; }  

6     public string Tags { get; set; }  

7  

8     public virtual ICollection<Post> Posts { get; set; }  

9 }

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对于包含导航属性的实体，如果要序列化最好关闭延迟加载

对于所有的实体都关闭延迟加载可以在context中设置

1 public class BloggingContext : DbContext 

2 { 

3     public BloggingContext() 

4     { 

5         this.Configuration.LazyLoadingEnabled = false; 

6     } 

7 }

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尽管延迟加载被禁用，仍然可以使用系显示加载去加载相关的实体

 1 using (var context = new BloggingContext()) 

 2 { 

 3     var post = context.Posts.Find(2); 

 4  

 5     // Load the blog related to a given post 

 6     context.Entry(post).Reference(p => p.Blog).Load(); 

 7  

 8     // Load the blog related to a given post using a string  

 9     context.Entry(post).Reference("Blog").Load(); 

10  

11     var blog = context.Blogs.Find(1); 

12  

13     // Load the posts related to a given blog 

14     context.Entry(blog).Collection(p => p.Posts).Load(); 

15  

16     // Load the posts related to a given blog  

17     // using a string to specify the relationship 

18     context.Entry(blog).Collection("Posts").Load(); 

19 }

20 //Reference放法在导航属性对应一个单独的实体时使用

21 //Collection 方法在导航属性对应一个实体集合时使用

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当显示加载相关实体时可以使用filters

 1 using (var context = new BloggingContext()) 

 2 { 

 3     var blog = context.Blogs.Find(1); 

 4  

 5     // Load the posts with the 'entity-framework' tag related to a given blog 

 6     context.Entry(blog) 

 7         .Collection(b => b.Posts) 

 8         .Query() 

 9         .Where(p => p.Tags.Contains("entity-framework") 

10         .Load(); 

11  

12     // Load the posts with the 'entity-framework' tag related to a given blog  

13     // using a string to specify the relationship  

14     context.Entry(blog) 

15         .Collection("Posts") 

16         .Query() 

17         .Where(p => p.Tags.Contains("entity-framework") 

18         .Load(); 

19 }

20 

21 using (var context = new BloggingContext()) 

22 { 

23     var blog = context.Blogs.Find(1); 

24  

25     // Count how many posts the blog has  

26     var postCount = context.Entry(blog) 

27                           .Collection(b => b.Posts) 

28                           .Query() 

29                           .Count(); 

30 }

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