How do I take last n elements from nested collection - asp.net-mvc

Question

I am buinding a chat for an application, so when a user logs in I need to send them all 'unseen' messages, I am using entityframework, Id like to return only the last 20 unseen messages. but my query is not working, currently I get this exception
Count must be a DbConstantExpression or a DbParameterReferenceExpression
what am I doing wrong?
List<ChatVM> unSeenChats = db.Chats.Where(chat => !chat.Seen)
.Select(chat => new ChatVM
{
Id = chat.Id,
IsAnnonymous = chat.IsAnnonymous,
UserName = chat.UserName,
Messages = chat.Messages
.OrderBy(x => x.DateTime)
.Skip(chat.Messages.Count - 20 > 0
? chat.Messages.Count - 20
: 0)
.Take(20)
.Select(message => new MessageVM
{
Id = message.Id,
DateTime = message.DateTime,
Text = message.Text
}).ToList()
}).ToList();
my models are as follows:
public class Chat
{
...
public virtual ICollection<Message> Messages { get; set; }
}
public class Message
{
...
public int ChatId { get; set; }
public virtual Chat Chat { get; set; }
}
public class Entities : IdentityDbContext<ApplicationUser>
{
....
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
...
modelBuilder.Entity<Message>()
.HasRequired(p => p.Chat).WithMany(p => p.Messages).WillCascadeOnDelete(true);
}
}
thanks

Answer 1

I don't think you're allowed to use .Count from within the query (hence the error that you're seeing). In any case, I think you're looking at this from the wrong perspective. You should probably be using the OrderByDescending method, and then just grab the first 20 posts from there.
Something like this:
List<ChatVM> unSeenChats = db.Chats.Where(chat => !chat.Seen)
.Select(chat => new ChatVM
{
Id = chat.Id,
IsAnnonymous = chat.IsAnnonymous,
UserName = chat.UserName,
Messages = chat.Messages
.OrderByDescending(x => x.DateTime)
.Take(20)
.Select(message => new MessageVM
{
Id = message.Id,
DateTime = message.DateTime,
Text = message.Text
}).ToList()
}).ToList();