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

 using (var context = new BloggingContext())
 {
     // Load all blogs and related posts
     var blogs1 = context.Blogs
                           .Include(b => b.Posts)
                           .ToList(); 

     // Load one blogs and its related posts
     var blog1 = context.Blogs
                         .Where(b => b.Name == "ADO.NET Blog")
                         .Include(b => b.Posts)
                         .FirstOrDefault(); 

     // Load all blogs and related posts
     // using a string to specify the relationship
     var blogs2 = context.Blogs
                           .Include("Posts")
                           .ToList(); 

     // Load one blog and its related posts
     // using a string to specify the relationship
     var blog2 = context.Blogs
                         .Where(b => b.Name == "ADO.NET Blog")
                         .Include("Posts")
                         .FirstOrDefault();
 }

预先加载也支持多层级加载相关的实体

 using (var context = new BloggingContext())
 {
     // Load all blogs, all related posts, and all related comments
     var blogs1 = context.Blogs
                        .Include(b => b.Posts.Select(p => p.Comments))
                        .ToList(); 

     // Load all users their related profiles, and related avatar
     var users1 = context.Users
                         .Include(u => u.Profile.Avatar)
                         .ToList(); 

     // Load all blogs, all related posts, and all related comments
     // using a string to specify the relationships
     var blogs2 = context.Blogs
                        .Include("Posts.Comments")
                        .ToList(); 

     // Load all users their related profiles, and related avatar
     // using a string to specify the relationships
     var users2 = context.Users
                         .Include("Profile.Avatar")
                         .ToList();
 }

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

 public class Blog
 {
     public int BlogId { get; set; }
     public string Name { get; set; }
     public string Url { get; set; }
     public string Tags { get; set; }  

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

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

 public class BloggingContext : DbContext
 {
     public BloggingContext()
     {
         this.Configuration.LazyLoadingEnabled = false;
     }
 }

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

 using (var context = new BloggingContext())
 {
     var post = context.Posts.Find(); 

     // Load the blog related to a given post
     context.Entry(post).Reference(p => p.Blog).Load(); 

     // Load the blog related to a given post using a string
     context.Entry(post).Reference("Blog").Load(); 

     var blog = context.Blogs.Find(); 

     // Load the posts related to a given blog
     context.Entry(blog).Collection(p => p.Posts).Load(); 

     // Load the posts related to a given blog
     // using a string to specify the relationship
     context.Entry(blog).Collection("Posts").Load();
 }
 //Reference放法在导航属性对应一个单独的实体时使用
 //Collection 方法在导航属性对应一个实体集合时使用

当显示加载相关实体时可以使用filters

 using (var context = new BloggingContext())
 {
     var blog = context.Blogs.Find(); 

     // Load the posts with the 'entity-framework' tag related to a given blog
     context.Entry(blog)
         .Collection(b => b.Posts)
         .Query()
         .Where(p => p.Tags.Contains("entity-framework")
         .Load(); 

     // Load the posts with the 'entity-framework' tag related to a given blog
     // using a string to specify the relationship
     context.Entry(blog)
         .Collection("Posts")
         .Query()
         .Where(p => p.Tags.Contains("entity-framework")
         .Load();
 }

 using (var context = new BloggingContext())
 {
     var blog = context.Blogs.Find(); 

     // Count how many posts the blog has
     var postCount = context.Entry(blog)
                           .Collection(b => b.Posts)
                           .Query()
                           .Count();
 }