LINQ search by userId from IQueryable

Question

I have two classes.

public class User
{
    public int UserId { get; set;}
    public string UserName { get; set; }
}

public class UserTimes
{
    public int UserId { get; set; }
    public DateTime Timestamp { get; set; }
}

Now I want to search for users based on recorded timestamps from UserTimes. I have my data from users as IQueryable<User> the UserTimes as IQueryable<UserTimes>

How can I find all users from my first IQueryable, where the userId is in my second IQueryable and the Timestamp is > xxxx

Answer 1

This can be achieved using a .Join in a linq query. The following is an example of retrieving all users with a time stamp greater than 2 days ago. Notice that only Huey and Dewey have time stamps that qualify; Louie only has older time stamps, thus the results will be as follows:

1::Huey

2::Duey

Here's the code listing:

void Main()
{
    var users = new List<User> { 
        new User { UserId = 1, UserName = "Huey" }, 
        new User { UserId = 2, UserName = "Dewey" },
        new User { UserId = 3, UserName = "Louie" }
    };

    var times = new List<UserTimes> {
        new UserTimes { UserId = 1, Timestamp = DateTime.Now.AddDays(-1) },
        new UserTimes { UserId = 1, Timestamp = DateTime.Now.AddDays(-2) },
        new UserTimes { UserId = 2, Timestamp = DateTime.Now.AddDays(-1) },
        new UserTimes { UserId = 2, Timestamp = DateTime.Now.AddDays(-2) },
        new UserTimes { UserId = 3, Timestamp = DateTime.Now.AddDays(-3) },
        new UserTimes { UserId = 3, Timestamp = DateTime.Now.AddDays(-3) },
    };
    
    var recentUsers = times
        .Join(users, t => t.UserId, u => u.UserId, (t, u) => new { User = u, Times = t })
        .Where(t => t.Times.Timestamp > DateTime.Now.AddDays(-2))
        .Select(t => t.User);

    recentUsers.ToList().ForEach(u => Console.WriteLine($"{u.UserId}::{u.UserName}"));
}

public class User
{
    public int UserId { get; set; }
    public string UserName { get; set; }
}

public class UserTimes
{
    public int UserId { get; set; }
    public DateTime Timestamp { get; set; }
}