EF Core：何时设置ID/对象？如何仅新增关联现有Book的Author

实体类定义 #

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();
}

尝试的操作代码 #

我尝试新增Author并关联现有Book（ID为1），代码如下：

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

_dbContext.Authors.Add(newAuthor);

异常现象 #

通过ChangeTracker.DebugView.LongView查看操作计划时，发现EF Core准备新增Author、Book和User三个实体：

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>

问题根源 #

问题出在Author类的Book属性默认初始化了new Book()。当调用_dbContext.Authors.Add(newAuthor)时，EF Core的ChangeTracker会自动发现关联的未被上下文跟踪的Book实例，将其标记为Added状态，进而尝试插入新的Book，连带触发关联的User新增。

正确解决方式 #

方式1：修正实体类初始化逻辑（推荐） #

将Author类中Book属性的默认初始化改为null，设置BookId时就不会触发新Book的追踪：

public class Author {
  // ...其他属性
  public Book? Book { get; set; } = null; // 改为null初始化
}

之后直接设置BookId即可：

var newAuthor = new Author();
newAuthor.BookId = 1;
newAuthor.Name = "My Author Name";
newAuthor.AddedByUserId = 1;

_dbContext.Authors.Add(newAuthor);
_dbContext.SaveChanges();

方式2：手动Attach现有Book（避免数据库查询） #

如果不想修改实体类，可以创建仅包含Id的Book实例，通过Attach将其标记为上下文已跟踪的现有实体，再关联到Author：

var existingBook = new Book { Id = 1 };
_dbContext.Attach(existingBook); // 标记为已跟踪的未修改实体

var newAuthor = new Author();
newAuthor.Book = existingBook; // 关联已跟踪的Book
newAuthor.Name = "My Author Name";
newAuthor.AddedByUserId = 1;

_dbContext.Authors.Add(newAuthor);
_dbContext.SaveChanges();

方式3：查询现有Book并关联（简单直观） #

就是你提到的方式，直接从数据库查询现有Book并赋值给Author的Book属性。这种方式会触发一次数据库查询，但逻辑最直观：

var existingBook = _dbContext.Books.Find(1);
if (existingBook != null)
{
    var newAuthor = new Author();
    newAuthor.Book = existingBook;
    newAuthor.Name = "My Author Name";
    newAuthor.AddedByUserId = 1;

    _dbContext.Authors.Add(newAuthor);
    _dbContext.SaveChanges();
}

内容的提问来源于stack exchange，提问作者user20326571