Linq外键| 如何仅显示具有特定外键的元素？

Question

I have two tables: Book and Chapter

Book
ID
Title
Author

Chapter
ChapterID
BookID
Title

I have a method in the Controller which is supposed to display a list of all the Chapters of a Book. However, it displays all the elements in the Chapter table, instead of the ones that match the BookID

Here is my method:

public ActionResult ViewChapters()
    {
        var chapters = db.Chapters
                             .Where(x => x.BookId ==  x.Book.ID)
                             .Include(x => x.Book).ToList();
        return View(chapters);
    }

I guess it's something wrong with my linq query.

Answer 1

You should pass the id of the book to the method, otherwise there's no way the code knows which book you're referencing.

public ActionResult ViewChapters(Guid bookId)
{
    var chapters = db.Chapters
                         .Where(x => x.BookId == bookId)
                         .Include(x => x.Book).ToList();
    return View(chapters);
}

The Where predicate evaluates each item in db.Chapters and only keeps the items where the predicate returns true. As Fabio mentions in the comments, when you compare x.BookId == x.Book.ID , the result is true for each item in the db.Chapters collection.