c# - INSERT 语句与 Entity Framework Core 中的 FOREIGN KEY 约束冲突

Question

我正在使用 Entity Framework 7 RC1，我有这些实体:

public class Book
{
    [Key]
    public string BookId { get; set; }
    public List<BookPage> Pages { get; set; }
    public Author Author { get; set; }
    public string Text { get; set; }
} 

public class BookPage
{
    [Key]
    public string BookPageId { get; set; }
    public int Number { get; set; }
}

public class Author
{
    [Key]
    public string AuthorId { get; set; }
    public string FullName { get; set; }
}

ApplicationDbContext.cs 是默认值，带有 public DbSet<Book> Books { get; set; }

当我尝试插入新书时，如下所示

var book = new Book()
{
    BookId = Guid.NewGuid().ToString(),
    Pages = new List<BookPage>()
    {
        new BookPage()
        {
            BookPageId = Guid.NewGuid().ToString(),
            Number = 1
        }
    },
    Author = new Author()
    {
        AuthorId = Guid.NewGuid().ToString(),
        FullName = "Forexample"
    },
    Text = "new book"
};
dbContext.Books.Add(book);
dbContext.SaveChanges();

它抛出异常:

SqlException: The INSERT statement conflicted with the FOREIGN KEY constraint "FK_Book_Author_AuthorAuthorId".

这个简单的插入有什么问题？我不想手动添加 BookPage 和 Author，这是 Entity Framework 的工作。

Answer 1

在您的 ApplicationDbContext 类中，您必须为 Author 类和 BookPage 类添加一个 DbSet :

public class ApplicationDbContext : DbContext
{
    public DbSet<Book> Books { get; set; }

    public DbSet<BookPage> BookPages { get; set; }

    public DbSet<Author> Authors { get; set; }

    ...
}

这样你就可以像这样重写你的代码:

var author = dbContext.Authors.Add(new Author()
{
    AuthorId = Guid.NewGuid().ToString(),
    FullName = "Forexample"
}).Entity;

var page = dbContext.BookPages.Add(new BookPage()
{
    BookPageId = Guid.NewGuid().ToString(),
    Number = 1
}).Entity;


var book = new Book.Book()
{
    BookId = Guid.NewGuid().ToString(),
    Pages = new List<BookPage>(),
    Text = "new book"
};
book.Pages.Add(page);
book.Author = author ;
dbContext.Books.Add(book);
dbContext.SaveChanges();

无论如何您不应该使用Guid作为主键(实际上作为聚集索引)，这是使用SQL Server的不好的做法。

您可以查看这篇文章以获取更多信息:

• What are the best practices for using a GUID as a primary key, specifically regarding performance?

另一种方法是使用标识列作为主键，并添加具有唯一约束的其他列(其中将包含唯一 ID)，以便您的模型看起来像这样:

public class Author
{
    public Author()
    {
        AuthorUid = Guid.NewGuid();
    }

    public int AuthorId { get; set; }
    public Guid AuthorUid { get; set; }
    public string FullName { get; set; }
}

public class Book
{
    public Book()
    {
        BookUid = Guid.NewGuid();
        Pages = new List<BookPage>();
    }

    public int BookId { get; set; }
    public Guid BookUid { get; set; }
    public List<BookPage> Pages { get; set; }
    public Author Author { get; set; }
    public string Text { get; set; }
}

public class BookPage
{
    public BookPage()
    {
        BookPageUid = Guid.NewGuid();
    }

    public int BookPageId { get; set; }
    public Guid BookPageUid { get; set; }
    public int Number { get; set; }
}

在 DbContext 中，您可以指定唯一约束:

public class BookContext : DbContext
{
    public DbSet<Book> Books { get; set; }

    public DbSet<BookPage> BookPages { get; set; }

    public DbSet<Author> Authors { get; set; }

    ...

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Author>().HasAlternateKey(a => a.AuthorUid);
        modelBuilder.Entity<Book>().HasAlternateKey(a => a.BookUid);
        modelBuilder.Entity<BookPage>().HasAlternateKey(a => a.BookPageUid);
    }
}

并像这样添加新实体:

using (var dbContext = new BookContext())
{
    var author = dbContext.Authors.Add(new Author()
    {
        FullName = "Forexample"
    }).Entity;

    var page = dbContext.BookPages.Add(new BookPage()
    {
        Number = 1
    }).Entity;

    var book = new Book.Book()
    {
        Text = "new book"
    };

    book.Pages.Add(page);
    book.Author = author;

    dbContext.Books.Add(book);
    dbContext.SaveChanges();
}