EFCore save entity with relation ids

Question

I'm trying to save an entity, but this entity is related to another one and of this one I only have the id. For example, giving this structure:

public class Library
{
    public int Id { get; set; }
    public string Name { get; set; }
    public ICollection<Book> Books { get; set; }
}

public class Book
{
    public int Id { get; set; }
    public int LibraryId { get; set; }
    public string Title { get; set; }
    public Bookshelf Bookshelf { get; set; }
}

public class Bookshelf
{
    public int Id { get; set; }
    public string Color { get; set; }
}

I receive from the client a new book to save to the db. This book only contains the Title, the LibraryId and the BookshelfId (it's a DTO). Now I want to save this book, creating the relationship with Library and Bookshelf, without retrieving the full object Library and Bookshelf, how can I do it easily?

What if, for example, I want to create a new library, and relate it to some already existing books? What I should do to achieve it?

Answer 1

I want to save this book, creating the relationship with Library and Bookshelf, without retrieving the full object Library and Bookshelf

Simply add a property BookshelfId on your book class.

public class Book
{
    public int Id { get; set; }
    public int LibraryId { get; set; }
    public string Title { get; set; }
    public Bookshelf Bookshelf { get; set; }
    public int BookshelfId { get; set; }
}

Then you can do:

DbContext.Add(new Book { LibraryId = 1, BookshelfId = 1});
DbContext.SaveChanges();

What if, for example, I want to create a new library, and relate it to some already existing books

Add Library navigation property on the book

public class Book
{
    public int Id { get; set; }
    public Library Library { get; set; }
    ...
}

and then you can update the library property on the books using only their ids

var library = new Library { Name = "My library" };
DbContext.Libraries.Add(library);

var bookIds = new int[] {1, 2};

foreach(var bookId in bookIds) {
    var book = new Book { Id= bookId, Library = library};
    DbContext.Attach(book);
    DbContext.Entry(book).Property(b => b.LibraryId).IsModified = true;
}

DbContext.SaveChanges();