Include collection in Entity Framework Core

Question

For example, I have those entities:

public class Book {     [Key]     public string BookId { get; set; }     public List<BookPage> Pages { get; set; }     public string Text { get; set; } }   public class BookPage {     [Key]     public string BookPageId { get; set; }     public PageTitle PageTitle { get; set; }     public int Number { get; set; } }  public class PageTitle {     [Key]     public string PageTitleId { get; set; }     public string Title { get; set; } } 

How should I load all PageTitles, if I know only the BookId?

Here it is how I'm trying to do this:

using (var dbContext = new BookContext()) {     var bookPages = dbContext         .Book         .Include(x => x.Pages)         .ThenInclude(x => x.Select(y => y.PageTitle))         .SingleOrDefault(x => x.BookId == "some example id")         .Pages         .Select(x => x.PageTitle)         .ToList(); } 

But the problem is, that it throws exception

ArgumentException: The properties expression 'x => {from Pages y in x select [y].PageTitle}' is not valid. The expression should represent a property access: 't => t.MyProperty'. When specifying multiple properties use an anonymous type: 't => new { t.MyProperty1, t.MyProperty2 }'. Parameter name: propertyAccessExpression

What's wrong, what exactly should I do?

Answer 1

Try accessing PageTitle directly in ThenInclude:

using (var dbContext = new BookContext()) {     var bookPages = dbContext     .Book     .Include(x => x.Pages)     .ThenInclude(y => y.PageTitle)     .SingleOrDefault(x => x.BookId == "some example id")     .Select(x => x.Pages)     .Select(x => x.PageTitle)     .ToList(); }