Question

我在ASP.NET MVC项目中遇到了逻辑问题。我正在开发一个看起来像Facebook的社交媒体应用。除了一件事，我得到了一切。

该应用程序有一个名为Follow的表，其中包含loggedInUser_ID和userToFollow_ID。我有一个IEnumerable个整数，其中包含登录用户的usersToFollow_ID列表，方法如下：

public IEnumerable<Post> postsForFlow(int userID, string userName)
{
    IEnumerable<int> userToFollowID = db.Follow.Where(a => 
        a.loggedInUser.Id == userID).Select(b => b.userToFollow.Id);

     return null;
}

如何预测这些整数以获取loggedInUser想要关注的所有帖子？

我可以使用Linq查询解决此问题吗？

Answer 1

这样的事情会起作用吗？这假设User与Posts有一对多的关系：

public IEnumerable<Post> PostsForFlow(int userId)
{
    var userIdsToFollow = db.Follow.Where(a =>
        a.loggedInUser.Id == userId).Select(b => b.userToFollow.Id);

    return db.Users
        .Where(u => userIdsToFollow.Contains(u.Id))
        .SelectMany(u => u.Posts);
}

修改

为了测试这一点，由于我没有你的数据库，我只是制作了一些虚拟课来嘲笑你的。如果这些与你的相似，那么这应该有效。

User表，其中包含Id和Post
列表
public class User { public int Id { get; set; } public List<Post> Posts { get; set; } }

Post表，其中只有Text属性（用于测试）

public class Post { public string Text { get; set; } }

Follow表，其中包含loggedInUser和userToFollow

public class Follow { public User loggedInUser { get; set; } public User userToFollow { get; set; } }

＆＃39; db＆＃39;，其中包含User列表和Follow列表（这些是表格）

public static class db { public static List<Follow> Follow = new List<Follow>(); public static List<User> Users = new List<User>(); }

然后我创建了一个用10个用户填充数据库的方法。用户ID＆＃39; 0＆＃39;是＆＃34;已登录的用户＆＃34;，用户1 - 9后面跟着用户0，每个帖子都有4个帖子

/// <summary> /// This populates the Db with 10 users. All users /// from 1 to 9 are being followed by user 0 /// </summary> public static void PopulateDb() { // Create the logged in user var loggedInUser = new User { Id = 0, Posts = new List<Post>() }; db.Users.Add(loggedInUser); // Create 9 other users with 4 posts each for (int i = 1; i < 10; i++) { var newUser = new User { Id = i, Posts = new List<Post> { new Post {Text = "Post #" + i}, new Post {Text = "Post #" + (i * 10)}, new Post {Text = "Post #" + (i * 20)}, new Post {Text = "Post #" + (i * 30)} } }; // Add the other user db.Users.Add(newUser); // Have the logged in user follow this new user db.Follow.Add(new Follow { loggedInUser = loggedInUser, userToFollow = newUser }); } }

然后，为了测试它，我只是调用两种方法：

public static void Main() { PopulateDb(); var postsForFlow = PostsForFlow(0); // Now postsForFlow has 36 entries - 4 for each // of the 9 users that user '0' is following Console.WriteLine(postsForFlow.Count()); // Output: 36 (9 users * 4 posts each) // Now, to test it further, stop following all users who have even Ids // This will leave us with 5 users: 1, 3, 5, 7, 9 foreach (var follow in db.Follow .Where(f => f.userToFollow.Id % 2 == 0) .ToList()) { db.Follow.Remove(follow); } postsForFlow = PostsForFlow(0); Console.WriteLine(postsForFlow.Count()); // Output: 20 (5 users * 4 posts each) }

Answer 2

只需添加对ToList（）的调用：

public IEnumerable<Post> postsForFlow(int userID, string userName)
{
    var userToFollowID = db.Follow.Where(a => 
                  a.loggedInUser.Id == userID).Select(b => b.userToFollow.Id).ToList();
    userToFollowID.ForEach(user =>
          {
                Follow(user); // Use whatever logic you need for following.  "user" is the int you want
          });
    return null;
}

Linq查询来自IEnumerable的整数

2 个答案: