EF 核心错误：“没有找到适合实体类型的构造函数……”

Question

我正在尝试通过 Entity Framework Core 创建数据库。

我的“域”项目中有以下模型：

public abstract class Note
{
    protected Note(string mainBody, DateTime publicationMoment, User author)
    {
        MainBody = mainBody;
        PublicationMoment = publicationMoment;
        AuthorId = author.Id;
        Author = author;
    }

    public int Id { get; set; }

    [Required]
    [MaxLength(1500)]
    public virtual string MainBody { get; set; }

    [Required]
    [NoteDateTime]
    public DateTime PublicationMoment { get; set; }

    [Required]
    public int AuthorId { get; set; }

    [Required]
    public User Author { get; set; }
}

public class Comment : Note
{
    public Comment(Post post, string mainBody, DateTime publicationMoment, User author) : 
        base(mainBody, publicationMoment, author)
    {
        Post = post;
        PostId = post.Id;
    }

    [Required]
    public int PostId { get; set; }

    [Required]
    public Post Post { get; set; }
}

public class Post : Note
{
    public Post(string title, string mainBody, DateTime publicationMoment, User author) : 
        base(mainBody, publicationMoment, author) => Title = title;
    
    [Required]
    [MaxLength(100)]
    public string Title { get; set; }

    public IList<Comment> Comments { get; set; } = new List<Comment>();
}

public class User
{
    public User(string nickname, int age, Gender gender)
    {
        Nickname = nickname;
        Age = age;
        Gender = gender;
    }

    public int Id { get; set; }

    [Required]
    [MaxLength(20)]
    public string Nickname { get; set; }

    [Required]
    [Range(14, 130)]
    public int Age { get; set; }

    [Required]
    public Gender Gender { get; set; }
}

我的“基础设施”项目中的 BlogContext 类：

public class BlogContext : DbContext
{
    public DbSet<User> Users => Set<User>();
    public DbSet<Post> Posts => Set<Post>();
    public DbSet<Comment> Comments => Set<Comment>();

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        var builder = new ConfigurationBuilder()
            .SetBasePath(Directory.GetCurrentDirectory())
            .AddJsonFile("appsettings.json");

        var config = builder.Build();

        optionsBuilder.UseSqlServer(config.GetConnectionString("Default"));
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<User>().HasAlternateKey(u => u.Nickname);
    } 
}

当我尝试在包管理器控制台中添加我的第一个迁移时（像这样：

Add-Migration BlogDbCreation

，我收到以下错误：

“找不到适合实体类型‘Comment’的构造函数。以下构造函数的参数无法绑定到实体类型的属性： 无法在“评论（发布帖子、字符串主体、DateTime publicationMoment、用户作者）”中绑定“帖子”、“作者” 请注意，只有映射属性可以绑定到构造函数参数。无法绑定到相关实体的导航，包括对拥有类型的引用。”

那么，我该如何解决呢？

Answer 1

1. 在构造函数中传递 id:s 而不是引用类型，让 EF 为您进行映射。

2. 您是否考虑过放弃“Note”继承以支持 Post 和 Comment 之间严格的一对多关系？

   public abstract class Entity
   {
     public Guid Id { get; set; }
     public DateTimeOffset Created { get; set; }
   }
   
   public class Comment : Entity
   {
     private Comment() { }
     public Comment(string mainBody, DateTime publicationMoment, Guid authorId)
       {
         MainBody = mainBody;
         PublicationMoment = publicationMoment;
         AuthorId = authorId;
       }
     public Post Post { get; set; } = null!;
     public string MainBody { get; set; }
     public DateTime PublicationMoment { get; set; }
     public Guid AuthorId { get; set; }
     public User Author { get; set; } = null!;
   }
   
   public class Post : Entity
   {
     private Post() { }
     public Post(string title, string mainBody, DateTime publicationMoment, Guid authorId)
      {
        Title = title;
        MainBody = mainBody;
        PublicationMoment = publicationMoment;
        AuthorId = authorId;
      }
   
     public string Title { get; set; }
     public string MainBody { get; set; }
     public DateTime PublicationMoment { get; set; }
     public Guid AuthorId { get; set; }
     public User Author { get; set; } = null!;
     public IList<Comment> Comments { get; set; } = new List<Comment>();
   }
   
    public class User : Entity
    {
      private User() { }
      public User(string nickname, int age)
        {
          Nickname = nickname;
          Age = age;
        }
     public string Nickname { get; set; } = null!;
     public int Age { get; set; }
    }