LINQ IQueryable Nested Loops - asp.net

Question

I am trying to replicate this loop as an IQueryable:
foreach (var book in BookList)
{
foreach (var a in book.Authors)
{
if (a.AuthorId.ToString() == Id)
{
AuthorView = new AuthorViewModel(a, item.BookId);
}
}
}
I have tried:
var Test = _context.Book.Include(x => x.Authors)
SelectMany(y => y.Authors).ToList().FirstOrDefault(x => x.AuthorId.ToString() == Id)
.Select(x => new AuthorViewModel(x, ??.BookId);
But I get stuck trying to create the object AuthorViewModel as it requires the BookId that I can no longer access. Is there a way of passing the book Id down?
Sorry, just release the example doesn't make sense.
Books is a table in a database.
Authors is a table in the database which contains many authors.
Book:
BookId
Authors
Authors:
AuthorId
Name
DOB
AuthorList should have been BookList (List). I have corrected this.
I am trying to locate an author based on an author id - and return it in an object (Author, BookId (from the book table))

Answer 1

This is I would do:
var id= int.Parse(Id);
var Test = _context.Book.Include(x => x.Authors)
.SelectMany(item => item.Authors
.Where(y => y.AuthorId == id)
.Select(b => new AuthorViewModel(b,item.BookId));
You can filter and project inside of the SelectMany extension method

Answer 2

I think you don't need SelectMany or Include at all. You can go like;
_context.Book
.Where(b => b.Authors.Any(a => a.AuthorId.ToString() == Id))
.Select(b => new AuthorViewModel(b.Authors.FirstOrDefault(x => x.AuthorId.ToString() == Id), b.BookId);

Answer 3

If you drill down on the Authors (SelectMany) then you have no way to go back to books. The condition on AuthorId needs to be handled internally.
var Test = _context.Book.Include(x => x.Authors)
.FirstOrDefault(x => x.Authors.Any(y => y.AuthorId.ToString() == Id))
.Select(x => new AuthorviewModel(x,x.BookId);
of course this will select only one book per author, just like your code. I wonder if your desired behaviour is to get all books instead...

Answer 4

First of all, convert Id to a number instead of trying to convert AuthorId to a string. Applying any kind of function on a table field means that indexes that cover it can't be used. Instead of a fast search based on an ID, you'll have a full table scan that converts values to strings before comparing them using string semantics. If the Id parameter contains any leading zeros, the comparison will fail.
As for the LINQ query itself, the easiest way is to use the query form. If you want to return one record for each book and author combination, you can write :
var id=int.Parse("ID");
var query= from book in _context.Book
from author in book.Authors
where author.AuthorID = id
select new AuthorViewModel(author,book.BookId);
var records=query.ToList();