Entity Framework Core で値オブジェクトを実装する

Entity Framework Core 2.0 の頃から既に値オブジェクトを実装する資料は存在した。

docs.microsoft.com

ただ、自分がやりたかったのは「エンティティの ID を値オブジェクトにすること」だったので、ちょっと違う。 Entity Framework Core 2.1 から追加された「値の変換」機能を使えば、やりたかったことが実現できるかもと思ったので試してみた。

using Microsoft.EntityFrameworkCore;
using System;
using System.Collections.Generic;
using System.Threading.Tasks;

namespace ValueObjectSample
{
    class Program
    {
        static void Main(string[] args)
        {
            try
            {
                MainAsync(args).GetAwaiter().GetResult();
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }
            Console.WriteLine("Press enter key");
            Console.ReadLine();
        }

        static async Task MainAsync(string[] args)
        {
            using (var context = new ApplicationDbContext())
            {
                await context.Database.EnsureDeletedAsync();
                await context.Database.EnsureCreatedAsync();
            }

            using (var context = new ApplicationDbContext())
            {
                var author = new Author()
                {
                    Name = "荒川 弘"
                };
                context.Authors.Add(author);

                context.Books.Add(new Book()
                {
                    Title = "銀の匙 Silver Spoon",
                    AuthorId = author.Id,
                });
                context.Books.Add(new Book()
                {
                    Title = "アルスラーン戦記",
                    AuthorId = author.Id,
                });

                await context.SaveChangesAsync();
            }

            using (var context = new ApplicationDbContext())
            {
                var books = await context.Books
                    .Include(b => b.Author)
                    .ToListAsync();
                foreach (var book in books)
                {
                    Console.WriteLine($"Id : {book.Id}");
                    Console.WriteLine($"Title : {book.Title}");
                    Console.WriteLine($"AuthorName : {book.Author.Name}");
                    Console.WriteLine();
                }
            }

            using (var context = new ApplicationDbContext())
            {
                var authors = await context.Authors
                    .Include(a => a.Books)
                    .ToListAsync();
                foreach (var author in authors)
                {
                    Console.WriteLine($"Id : {author.Id}");
                    Console.WriteLine($"Name : {author.Name}");
                    for (int i = 0; i < author.Books.Count; i++)
                    {
                        Console.WriteLine($"BookTitle{i} : {author.Books[i].Title}");
                    }
                    Console.WriteLine();
                }
            }
        }
    }

    public class ApplicationDbContext : DbContext
    {
        public DbSet<Author> Authors => Set<Author>();

        public DbSet<Book> Books => Set<Book>();

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseSqlServer(
                "Server=(local);Database=ValueObjectSample;Integrated Security=SSPI;");
        }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Author>(b =>
            {
                b.Property(e => e.Id)
                    .IsRequired()
                    .HasConversion( // データベースには文字列で保存する
                        id => id.Value,
                        value => AuthorId.Parse(value));
                b.Property(e => e.Name)
                    .IsRequired();

                b.HasKey(e => e.Id);
                b.HasMany(e => e.Books);
            });

            modelBuilder.Entity<Book>(b =>
            {
                b.Property(e => e.Id)
                    .IsRequired()
                    .HasConversion( // データベースには文字列で保存する
                        id => id.Value,
                        value => BookId.Parse(value));
                b.Property(e => e.Title)
                    .IsRequired();
                b.Property(e => e.AuthorId)
                    .IsRequired()
                    .HasConversion( // データベースには文字列で保存する
                        id => id.Value,
                        value => AuthorId.Parse(value));

                b.HasKey(e => e.Id);
                b.HasOne(e => e.Author);
            });
        }
    }

    public class Author
    {
        public AuthorId Id { get; set; } = AuthorId.NewAuthorId();

        public string Name { get; set; }

        public IList<Book> Books { get; set; }
    }

    public class Book
    {
        public BookId Id { get; set; } = BookId.NewBookId();

        public string Title { get; set; }

        public AuthorId AuthorId { get; set; }

        public Author Author { get; set; }
    }

    // Author の ID を ValueObject にする
    public class AuthorId
    {
        AuthorId(string value)
        {
            Value = value;
        }

        public string Value { get; }

        public static AuthorId NewAuthorId() =>
            new AuthorId(Guid.NewGuid().ToString());

        public static AuthorId Parse(string value) =>
            new AuthorId(value);

        public override int GetHashCode() =>
            Value.GetHashCode();

        public override bool Equals(object obj)
        {
            if (obj == null || GetType() != obj.GetType())
                return false;
            if (object.ReferenceEquals(this, obj))
                return true;
            if (obj is AuthorId other)
                return Value.Equals(other.Value);
            return false;
        }

        public override string ToString() =>
            $"AuthorId({Value})";
    }

    // Book の ID を ValueObject にする
    public class BookId
    {
        BookId(string value)
        {
            Value = value;
        }

        public string Value { get; }

        public static BookId NewBookId() =>
            new BookId(Guid.NewGuid().ToString());

        public static BookId Parse(string value) =>
            new BookId(value);

        public override int GetHashCode() =>
            Value.GetHashCode();

        public override bool Equals(object obj)
        {
            if (obj == null || GetType() != obj.GetType())
                return false;
            if (object.ReferenceEquals(this, obj))
                return true;
            if (obj is BookId other)
                return Value.Equals(other.Value);
            return false;
        }

        public override string ToString() =>
            $"BookId({Value})";
    }
}