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(2); 
 
    // 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(1); 
 
    // 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(1); 
 
    // 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(1); 
 
    // Count how many posts the blog has  
    var postCount = context.Entry(blog) 
                          .Collection(b => b.Posts) 
                          .Query() 
                          .Count(); 
}