Result of left joining a nullable nominal type should be null (i.e. no instance) and not an instance with all null property values

when Blog does not have a Post, following query does not work in 5.0.0-preview.8.* or 6.0.0-* nightly. but works in 5.0.0-preview.7.*

 public class Blog
    {
        public int BlogId { get; set; }
        public string Url { get; set; }

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

        public List<Post> Posts { get; set; } = new List<Post>();
    }

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

        public string Title { get; set; }

        public string Content { get; set; }

        public int BlogId { get; set; }

        public Blog Blog { get; set; }
    }

// this IQueryable would come from other API.

 var dbPosts = from p in db.Posts
                          // select p;
                         select new Post
                         {
                             PostId = p.PostId,
                             BlogId = p.BlogId,
                             Content = p.Content
                         };

 var query = from blog in db.Blogs
                    join post in dbPosts on blog.BlogId equals post.BlogId into posts
                    from xpost in posts.DefaultIfEmpty()
                    select new Blog
                    {
                         Url = blog.Url,
                         Post = xpost
                   };

Steps to reproduce

I have a repo to reproduce the bug.

https://github.com/skclusive/EFLeftJoinBug

Unhandled exception. System.InvalidOperationException: Nullable object must have a value.
   at lambda_method17(Closure , QueryContext , DbDataReader , ResultContext , SingleQueryResultCoordinator )
   at Microsoft.EntityFrameworkCore.Query.Internal.SingleQueryingEnumerable`1.Enumerator.MoveNext()

Further technical details

EF Core version: Database provider: (e.g. Microsoft.EntityFrameworkCore.Sqlite) Target framework: (e.g. .NET Core 5.0) Operating system: IDE: (e.g. Visual Studio Code)

[optiks]

We’ve just hit this after (attempting) to upgrade from 3.1 to 5.0. From what I can tell, this is a significant behavioural change.

Suggestion: Mention this is Breaking changes in EF Core 5.0?

Googling stack traces isn’t much fun 😃

[Shiko1st]

This is definitely a breaking change