c# - 如何在 Entity Framework Core 中配置多对多关系

Question

我有一些文章和一些作者，想将它们链接为:

• 任何文章都可以有任意数量的作者和
• 每个作者都可以写任意数量的文章

我使用本教程创建了多对多关系:https://www.learnentityframeworkcore.com/configuration/many-to-many-relationship-configuration

这是我定义类的方式。

文章

public class Article
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int ID { get; set; }

    public string Title { get; set; }

    public DateTime Date { get; set; }

    public ICollection<ArticleAuthor> ArticleAuthors { get; set; }
}

作者

public class Author
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int ID { get; set; }

    [Column(TypeName = "text")] public string Name { get; set; }

    [Column(TypeName = "text")] public string Email { get; set; }

    public ICollection<ArticleAuthor> ArticleAuthors { get; set; }
}

文章作者

public class ArticleAuthor
{
    public int ArticleId { get; set; }

    public Article Article { get; set; }

    public int AuthorId { get; set; }

    public Author Author { get; set; }
}

数据库上下文

public class AppDbContext : DbContext
{
    public AppDbContext(DbContextOptions<AppDbContext> options) : base(options)
    {
    }

    public DbSet<Article> Articles { get; set; }

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

    public DbSet<ArticleAuthor> ArticleAuthors { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<ArticleAuthor>()
            .HasKey(bc => new {bc.ArticleId, bc.AuthorId});
        modelBuilder.Entity<ArticleAuthor>()
            .HasOne(bc => bc.Article)
            .WithMany(b => b.ArticleAuthors)
            .HasForeignKey(bc => bc.ArticleId);
        modelBuilder.Entity<ArticleAuthor>()
            .HasOne(bc => bc.Author)
            .WithMany(c => c.ArticleAuthors)
            .HasForeignKey(bc => bc.AuthorId);
    }
}

在我运行 dotnet ef database update 后，表按预期创建了

一切似乎都很好，除了当我尝试通过此服务添加带有作者的文章时:

服务

// POST: api/Articles
[HttpPost]
public async Task<IActionResult> PostArticle([FromBody] Article article)
{
    if (!ModelState.IsValid)
    {
        return BadRequest(ModelState);
    }

    _context.Articles.Add(article);
    await _context.SaveChangesAsync();

    return Ok(article);
}

JSON

{
    "Title":"Title 2",
    "ReadingTime" : 2,
    "Date":"01.12.2019",
    "Theme": "1",
    "ArticleAuthors":[
        {
            "ID":"1"
        }
    ]
}

当我尝试添加有作者的文章时，我一直卡在这个错误中

System.InvalidOperationException: The property 'AuthorId' on entity type 'ArticleAuthor' has a temporary value. Either set a permanent value explicitly or ensure that the database is configured to generate values for this property.

Answer 1

试试这个payload，ArticleId和AuthorId应该是必须的，ArticleId在父类中是隐含的，所以你至少应该提供AuthorId:

{
    "Title":"Title 2",
    "ReadingTime" : 2,
    "Date":"01.12.2019",
    "Theme": "1",
    "ArticleAuthors":[
        {
            "AuthorId":"1"
        }
    ]
}