EF Core 3.18 get sum and count from related table - .net-core

Question

I have a web api where I am trying to get sum and count of a related table. Using .net core 3 and EF Core 3.1.8.
This is what I have tried:
_context.Books
.Include(r => r.BookCategories)
.Include(r => r.Resources)
.Include(r => r.Ratings.GroupBy(g => g.Bookid).Select(s => new { SumAllVotes = s.Sum(item => item.Rating) }))
.ToListAsync();
But I just get an error message. (see below).
I find it difficault debugging with EF Core as I dont know where it is going wrong. Have been trying a couple of hours, but whatever I write I get the same error message.
Thought maybe you guys were able to see what was wrong.
What I want
I am trying to get Sum of all Rating inside table Ratings.
Rating contains only 0 or 1. And I am trying to sum ratings on each bookid. I wanted to have it in this class public int SumAllVotes { get; set; }.
Because I list out all Books...and one of the properties will then be SumAllVotes. (And also CountAllVotes, when I have finished this problem).
By the end I will have a SumAllVotes and CountAllVotes and can calculate the percentage of how many have pressed "1".
Error message:
An unhandled exception occurred while processing the request.
InvalidOperationException: Lambda expression used inside Include is
not valid.
Microsoft.EntityFrameworkCore.Query.Internal.NavigationExpandingExpressionVisitor.ProcessInclude(NavigationExpansionExpression
source, Expression expression, bool thenInclude)
What I have tried:
[HttpGet]
public async Task<ActionResult<IEnumerable<Books>>> GetBooks()
{
Guid userid = Guid.Parse(this.User.FindFirst(ClaimTypes.NameIdentifier).Value);
return await _context.Books
.Include(r => r.BookCategories)
.Include(r => r.Resources)
.Include(r => r.Ratings.GroupBy(g => g.Bookid).Select(s => new { SumAllVotes = s.Sum(item => item.Rating) }))
.ToListAsync();
}
Books and Ratings are defined as -
public partial class Books
{
public Books()
{
Bookmarks = new HashSet<Bookmarks>();
Comments = new HashSet<Comments>();
Favourites = new HashSet<Favourites>();
BookCategories = new HashSet<BookCategories>();
Resources = new HashSet<Resources>();
Ratings = new HashSet<Ratings>();
}
public int Id { get; set; }
public string Title { get; set; }
public string Description { get; set; }
public byte Scaleoffun { get; set; }
public byte Scaleoflearning { get; set; }
public int? Goal { get; set; }
public int? Secondgoal { get; set; }
public int? Thirdgoal { get; set; }
public int? Subjectid { get; set; }
public int? Categoryid { get; set; }
public string Language { get; set; }
public string Estimatedtime { get; set; }
public string Image { get; set; }
public int? File { get; set; }
public int? Ownerid { get; set; }
public DateTime Createdon { get; set; }
public DateTime? Lastmodifiedon { get; set; }
public string Active { get; set; }
public string Url { get; set; }
public Guid Userid { get; set; }
public byte? Grade { get; set; }
[NotMapped]
public int SumAllVotes { get; set; }
[NotMapped]
public int CountAllVotes { get; set; }
public virtual Categories Category { get; set; }
public virtual Curriculum GoalNavigation { get; set; }
public virtual Users Owner { get; set; }
public virtual Curriculum SecondgoalNavigation { get; set; }
public virtual Subjects Subject { get; set; }
public virtual Curriculum ThirdgoalNavigation { get; set; }
public virtual ICollection<Bookmarks> Bookmarks { get; set; }
public virtual ICollection<Comments> Comments { get; set; }
public virtual ICollection<Favourites> Favourites { get; set; }
public virtual ICollection<BookCategories> BookCategories { get; set; }
public virtual ICollection<Resources> Resources { get; set; }
public virtual ICollection<Ratings> Ratings { get; set; }
}
public partial class Ratings
{
public int Id { get; set; }
public int? Bookid { get; set; }
public string Type { get; set; }
public Int16? Rating { get; set; }
public Guid Userid { get; set; }
public string Subject { get; set; }
public DateTime Createdon { get; set; }
public DateTime? Modifiedon { get; set; }
public byte? Active { get; set; }
public virtual Books Book { get; set; }
//public virtual Users User { get; set; }
}
These are some other solutions I have tried, but got the same error message:
.Include(r=> r.Ratings.Sum(i=>i.Rating))
and
.Include(r => new { m = r.Ratings.GroupBy(g => g.Bookid) })

Answer 1

You don't need to group child entities by parent's Id. When you Include one-to-many child entities, they are added to their parent's child list, and hence grouped by their parent's identity, based on the relationship between them. All you need to do is tell EF what values you want from that child list.
Change your query to -
_context.Books
.Include(r => r.BookCategories)
.Include(r => r.Resources)
.Include(r => r.Ratings)
.Select(p => new
{
// set ALL the primitive properties from Books entity
Id = p.Id,
Title = p.Title,
// etc ...
// set the computed properties
CountAllVotes = p.Ratings.Count,
SumAllVotes = p.Ratings.Sum(x => x.Rating)
// set the related entities
BookCategories = p.BookCategories,
Resources = p.Resources
})
.ToListAsync();
AutoMapper has a ProjectTo method that generates the required query and does the projection (the Select part) automatically. You can use that to avoid the hassle of setting all those properties manually.

Answer 2

I suggest you don't use Include with Select. Read article how to make queries with Projection (Select). Note, that Rating.Rating is nullable and you need to handle this. Here is a possible code sample:
var view = await _context.Books
.Where(your condition)
.Select(item => new
{
//Todo: fill other props
SumAllVotes = item.Ratings.Sum(rating => (Int16?) rating.Rating),
CountAllVotes = item.Ratings.Count,
})
.ToListAsync()