I have the following entities:

public class Book {
  public int Id { get; set; }
  public string Name { get; set; } = string.Empty;
  public int? AddedByUserId { get; set; }

  public virtual ICollection<Author> Authors { get; set; } = new HashSet<Author>();
}


public class Author {
  public int Id { get; set; }
  public int BookId { get; set; }
  public string Name { get; set; } = string.Empty;
  public int? AddedByUserId { get; set; }

  public Book Book { get; set; } = new Book();
}

I try to add an Author and set the BookId to an existing value.

var newAuthor = new Author();
newAuthor.BookId = 1;

_dbContext.Authors.Add(author);

When I inspect the ChangeTracker to see what DbContext is going to do:

var longView = _dbContext.ChangeTracker.DebugView.LongView;

It indicates that Author, Book, and User will all be added.

Author {Id: -2147482646} **Added**
  Id: -2147482646 PK Temporary
  AddedByUserId: 1
  DateAdded: '11/25/2022 8:22:11 PM'
  Name: 'My Author Name'
  BookId: -2147482643 FK Temporary
  Book: {Id: -2147482643}
Book {Id: -2147482643} **Added**
  Id: -2147482643 PK Temporary
  AddedByUserId: -2147482645 FK Temporary
  DateAdded: '1/1/0001 12:00:00 AM'
  Name: ''
  AddedByUserId: {Id: -2147482645}
User {Id: -2147482645} **Added**
  Id: -2147482645 PK Temporary
  DateAdded: '1/1/0001 12:00:00 AM'
  Name: <null>
  RowVersion: <null>

How can I do it so that only the new Author gets added when the BookId foreign key is set? Should I be setting objects instead of Ids?

newAuthor.Book = _dbContext.Book.Find(1);

CodePudding user response：

newAuthor.Book is not null and is not tracked by EF so it will consider it as a new book. There are multiple options:

1. "Clear" the book object:

var newAuthor = new Author();
newAuthor.BookId = 1;
newAuthor.Book = null;

2. Attach the book:

var newAuthor = new Author();
newAuthor.Book.Id = 1;
_dbContext.Books.Attach(newAuthor.Book);

_dbContext.Authors.Add(author);

3. Fetch the book from database (as you have suggested yourself)

CodePudding user response：

Yes, don't modify the foreign keys directly, instead let ef core do its job by setting the object itself. Also don't new() reference navigation properties:

public Book Book { get; set; } = new Book();

doing it for collection navigation properties is good because it avoids null exceptions when trying to add to the collection, for example.