Cannot convert type 'System.Linq.IQueryable<int>' to 'int'

I have a problem sending data from the database to a list in the Controller. I am new to C# and MVC so any help will be useful! Code follows

public static List<FilterTree> GetAllUsers()
    {
        FilterTreeDBContext db = new FilterTreeDBContext();
        var userList = new List<FilterTree>();
        var device =  new FilterTree();
        //This is the line where I get the error
        device.ID = from a in db.FilterTree select a.ID;

        userList.Add(device);
        return userList;
    }

Thanks & Happy Holidays!! :)