How to use .Include() in EF Core with ViewModel - asp.net-core

Question

I have two models
public class PageGroup
{
public PageGroup()
{
Pages = new HashSet<Page>();
}
public int GroupID { get; set; }
public string GroupTitle { get; set; }
public virtual ICollection<Page> Pages { get; set; }
}
public class Page
{
public Page()
{
}
public int PageID { get; set; }
public int GroupID { get; set; }
public string PageTitle { get; set; }
public string PageText { get; set; }
public virtual PageGroup PageGroup { get; set; }
}
and a ViewModel
public class ShowGroupsViewModel
{
public int GroupID { get; set; }
public string GroupTitle { get; set; }
public int PageCount { get; set; }
}
I filled this ViewModel with this method
public async Task<IEnumerable<ShowGroupsViewModel>> GetListGroupsServiceAsync()
{
return await _context.PageGroups.Include(p => p.Pages.Count).Select(pa => new ShowGroupsViewModel()
{
GroupID = pa.GroupID,
GroupTitle = pa.GroupTitle,
PageCount = pa.Pages.Count
}).ToListAsync();
}
but PageCount is not work. when run the project is has zero value. How can I fill this property?
I use .net core 3.1

Answer 1

Since you are using Ef 3.1 Include should not be used because Include("Pages") will bring all Page instances from the SQl server and count them after this.
In EF Net5 it would be done much more simple, using Include, but since you are using EF 3.1 try this:
public async Task<IEnumerable<ShowGroupsViewModel>> GetListGroupsServiceAsync()
{
return await ( from pg in _context.PageGroups
join p in context.Pages
on pg.GroupId equals p.GroupId
group pg by new { pg.GroupId, pg.GroupTitle} into g
select new ShowGroupsViewModel{
GroupId = g.Key.GroupId,
GroupTitle =g.Key.GroupTitle
PagesCount = g.Count()
}).ToListAsync();
}