EF中加载实体的方式

EF中的查询执行时机：
1. foreach进行枚举
2. ToArray、ToList、ToDictionary
3. Linq的一些操作，如First、Any
4. DbSet上的Load操作。DbEntityEntry.Reload和Database.ExecuteSqlCommand

在web application中，每一个请求使用一个context实例；在WPF中，每个form使用一个context实例
context不是线程安全的

加载实体的方式：

1.贪婪加载(eager loading)

2.延迟加载(lazy loading)

3.显示加载(explicit loading)

贪婪加载实现是通过include方法实现的

 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();
 }

延迟加载通过virtual关键字进行实现（当EF框架禁用了延迟加载，这种方式就无效了）

 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; }
 }

 //禁用延迟加载
 public class BloggingContext : DbContext
 {
     public BloggingContext()
     {
         this.Configuration.LazyLoadingEnabled = false;
     }
 }

当禁用了延迟加载后或不使用延迟加载时，通过显示加载仍然可以获取管理的数据

 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();

 //一对多时使用Collection
     // 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();
 }

 //使用Query方式进行一些过滤
 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();
 }