Question

My repository layer used to return a ViewModel directly e.g.

public IEnumerable<CommentViewModel> GetComments()
{
   return from c in context.Comments
                 select new CommentViewModel
                 {
                    FirstName = c.FirstName,
                    Comment = c.Comment
                 };
}

I then read that the repository should not return view models so I changed it to:

public IEnumerable<Comment> GetComments()
{
   return from c in context.Comments;
}

In the service layer I then changed:

public IEnumerable<CommentViewModel> GetComments(int postId)
{
   return _repository.GetComments();
}

To:

public IEnumerable<CommentViewModel> GetComments()
{
   var comments = _repository.GetComments();

   return Mapper.Map<IEnumerable<Comment>, IEnumerable<CommentViewModel>>(comments);
}

The problem is the queries used to take 7ms and now take 85ms!

Where am I going wrong?

The query generated used to be (top 5 and order by is done in controller):

SELECT TOP (5) [t0].[FirstName], [t0].[Comment]
FROM [dbo].[Comment] AS [t0]
ORDER BY [t0].[CreateDate] DESC 

It’s now returning all columns:

SELECT 
[Extent1].[Id] AS [Id], 
[Extent1].[FirstName] AS [FirstName], 
[Extent1].[LastName] AS [LastName], 
[Extent1].[DatePosted] AS [DatePosted], 
[Extent1].[Comment] AS [Comment],
[Extent1].[IPAddress] AS [IPAddress]
FROM [dbo].[Comment] AS [Extent1]

Answer 1

You need to shape the query results in your repository layer otherwise AutoMapper will pull all the records from database to map to view models.

public IEnumerable<Comment> GetComments(int skip, int count)
{
   return context.Comments.OrderByDescending(c => c.CreateDate).Skip(skip).Take(count);
}