Seed data in EF 2.1 with related entities

To seed these posts I believe I need to:

Get the correspondent Category Id for each CategoryName;

Generate an ID for each post.

Yes! You are correct! you have to use CategoryId instead of CategoryName in the Post because CategoryId is the relational Key, not CategoryName. So your YML content should be like as follows:

new {Id = 1, CategoryId = 1, Title = "A", Content = "A Content" },
new {Id = 2, CategoryId = 2, Title = "B", Content = "B Content" }

Then in the OnModelCreating of the DbConext as follows:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
      base.OnModelCreating(modelBuilder);

      var posts = _service.GetPostsFromYMLFiles(path);

      modelBuilder.Entity<Category>().HasData(
          new { Id = 1, Name = "Reading"},
          new { Id = 2, Name = "Travelling" }
      );

      modelBuilder.Entity<Post>().HasData(posts);        
}

These YML files are created by a third party and I cannot change them.

Then you have to write a custom service method where you will check every post in YMl file with CategoryName then you have to make new list of Post where you will replace CategoryName with CateogryId and its appropriate value as follows:

public class YmlPost
{
    public string CategoryName { get; set; }
    public string Title { get; set; }
    public string Content { get; set; }
}


public class SeedDataService
{
    static List<Category> categoryList = new List<Category>()
    {
        new Category { Id = 1, Name = "Reading" },
        new Category { Id = 2, Name = "Traveling" }
    };

    public static List<Category> GetCategoriesForSeeding()
    {
       return categoryList;
    }
    public static List<Post> GetPostsForSeeding()
    {
        List<YmlPost> postListFromYML = _service.GetPostsFromYMLFiles(path);

        List<Post> posts = postListFromYML.Select((p, i) => new Post
            { 
              Id = i+1, // index is 0 based that's why you have to add 1.
              Title = p.Title,
              Content = p.Content,
              CategoryId = categoryList.Single(c => c.Name == 
                           p.CategoryName).Id
            }).ToList();
        return posts;
    }
}

Then in the OnModelCreating of the DbConext as follows:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
      base.OnModelCreating(modelBuilder);

      var categories = SeedDataService.GetCategoriesForSeeding();
      modelBuilder.Entity<Category>().HasData(categories);

      var posts = SeedDataService.GetPostsForSeeding();
      modelBuilder.Entity<Post>().HasData(posts);

}