今天在博問中看到一個關於 EF Core 的提問 ef core 2.0 多對多查詢的問題，由於還沒使用過 EF Core 的多對多映射，於是參考 EF Core 幫助文檔快速寫了個 .net core 控制臺程序（基於 EF Core In-Memory Database）實驗了一下。

實體類的定義：

1）Post

public class Post
{
    public int PostId { get; set; }
    public string Title { get; set; }
    public string Content { get; set; }

    
 
public List<PostTag> PostTags { get; set; }
}

2）Tag

public class Tag
{
    public int TagId { get; set; }
    public string TagName { get; set; }

    public List<PostTag> PostTags { get; set; }
}

3）PostTag

public class PostTag
{
    public int PostId { get; set; }
    
 
public Post Post { get; set; }

    public int TagId { get; set; }
    public Tag Tag { get; set; }
}

DbContext 的定義與映射配置：

public class MyDbContext : DbContext
{
    public DbSet<Post> Posts { get; set; }
    public DbSet<Tag> Tags { get; set; }

    public MyDbContext(DbContextOptions<MyDbContext> options)
        : 
 
base(options)
    { }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<PostTag>()
            .HasKey(t => new { t.PostId, t.TagId });

        modelBuilder.Entity<PostTag>()
            .HasOne(pt => pt.Post)
            .WithMany(p => p.PostTags)
            .HasForeignKey(pt => pt.PostId);

        modelBuilder.Entity<PostTag>()
            .HasOne(pt => pt.Tag)
            .WithMany(t => t.PostTags)
            .HasForeignKey(pt => pt.TagId);
    }
}

控制臺程序 Main 方法：

class Program
{
    static async Task Main(string[] args)
    {
        IServiceCollection services = new ServiceCollection();
        services.AddDbContext<MyDbContext>(options =>
        {
            options.UseInMemoryDatabase("blog_sample");
        });

        IServiceProvider sp = services.BuildServiceProvider();

        var writeDbContext = sp.GetService<MyDbContext>();

        var post = new Post
        {
            Title = "test title",
            Content = "test body",
            PostId = 1
        };
        writeDbContext.Add(post);

        var tag = new Tag
        {
            TagId = 2,
            TagName = "efcore"
        };
        writeDbContext.Add(tag);

        var postTag = new PostTag
        {
            PostId = 1,
            TagId = 2
        };
        writeDbContext.Add(postTag);
        writeDbContext.SaveChanges();

        var readDbContext = sp.GetService<MyDbContext>();
        post = await readDbContext.Posts.FirstOrDefaultAsync();
        Console.WriteLine(post.PostTags.FirstOrDefault().Tag.TagId); 
        //output is 2
    }
}

查詢時連 Include 都不需要，EF Core 會自動打理好一切。

初試 Entity Framework Core 的多對多映射