实体框架 - 渴望加载相关实体的相关实体

Question

我正在用实体框架5编写C＃（.NET 4.5）中的简单数据库应用程序。我遇到了一种情况，我可能或可能不需要加载单个实体的相关实体。如果碰巧需要加载实体的相关实体，我想急于加载相关实体的相关实体。基本上，我试图避免“SELECT N + 1”问题。希望下面的代码将使我想要做的事情明确：

using (var ctx = new DbContext()) 
{ 
    // Find a single instance of a person. I don't want to eagerly load 
    // the person's friends at this point because I may not need this 
    // information. 
    var person = ctx.Persons.Single(x => x.PersonID == 12); 

    // Many lines of code... 
    // Many lines of code... 
    // Many lines of code... 

    // Okay, if we have reached this point in the code, the person wants to 
    // send all his friends a postcard, so we need the person's friends and 
    // their addresses. 

    // I want to eagerly load the person's friends' addresses, but the Include 
    // method is not allowed. The property "Friends" is just an ObservableCollection. 
    var friends = person.Friends.Include("Address").ToList(); 

    // So, I must do the following: 
    var friends = person.Friends.ToList(); 

    // Now I will output the address of each friend. This is where I have the 
    // SELECT N+1 problem. 
    foreach(var friend in friends) 
    { 
     // Every time this line is executed a query (SELECT statement) is sent 
     // to the database. 
     Console.WriteLine(friend.Address.Street); 

    } 

} 

任何想法，我应该做什么？

Answer 1

这对明确装载的良好局面 - 第三个选项加载相关实体与实体框架之外急切和懒惰加载：

using (var ctx = new DbContext()) 
{ 
    var person = ctx.Persons.Single(x => x.PersonID == 12); 

    // ... 

    // the following issues one single DB query 
    person.Friends = ctx.Entry(person).Collection(p => p.Friends).Query() 
     .Include(f => f.Address) // = .Include("Address") 
     .ToList(); 

    foreach(var friend in person.Friends) 
    { 
     // No DB query because all friends including addresses 
     // have already been loaded 
     Console.WriteLine(friend.Address.Street); 
    } 
} 

这里的关键是.Query()它返回一个可查询的Friends集合并允许您为朋友收藏添加任意附加查询逻辑 - 例如筛选，订购，加入其他相关数据（= Include），聚合（例如Count朋友）等。