何时设置Id，何时设置Object

Question

我有以下实体:

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();
}

我尝试添加Author并将BookId设置为现有值。

var newAuthor = new Author();
newAuthor.BookId = 1;
_dbContext.Authors.Add(author);

当我检查ChangeTracker，看看DbContext要做什么:

var longView = _dbContext.ChangeTracker.DebugView.LongView;

表示添加Author、Book、User。

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>

当设置BookId外键时，如何才能只添加新的Author?我应该设置对象而不是id吗?

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

Answer 1

newAuthor.Book不是空的，EF不会跟踪它，所以它会把它当作一本新书。有多个选项:

1. "Clear"图书对象:

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

2. 附加图书:

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

3. 从数据库中获取图书(如您自己所建议的)

Answer 2

是的，不要直接修改外键，而是让ef core通过设置对象本身来完成它的工作。也不要引用new()导航属性:

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

为集合导航属性这样做是很好的，因为它可以避免在尝试添加到集合时出现null异常，例如