Query: Optimize navigation property translation in projections

For example, customers.Select(c => new { c.Id, Orders = c.Orders }) results in n+1 at the moment.

Make sure to consider the following scenarios when looking at this:

• https://github.com/aspnet/EntityFramework/issues/5754
• https://github.com/aspnet/EntityFramework/issues/5738
• https://github.com/aspnet/EntityFramework/issues/5791

[maumar]

@MaklaCof I’m working on this feature right now. Its scheduled to be shipped with the next release of EF Core. Unless I encounter some unexpected issues the code should be in our dev branch in a next couple weeks or so - ready to be tested

[maumar]

@simeyla here is a sample of how a manually created query (using joins) would look like:

        public class MyContext : DbContext
        {
            public DbSet<Email> Emails { get; set; }

            public DbSet<Tag> Tags { get; set; }

            protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
            {
                optionsBuilder.UseSqlServer(@"Server=.;Database=Sample4007;Trusted_Connection=True;MultipleActiveResultSets=True");
            }
        }

        public class EmailDto
        {
            public string Subject { get; set; }

            public List<TagDto> Tags { get; set; }
        }

        public class TagDto
        {
            public int Id { get; set; }
            public string Name { get; set; }
        }

        public class Email
        {
            public int Id { get; set; }
            public string Subject { get; set; }

            public List<Tag> Tags { get; set; }
        }

        public class Tag
        {
            public int Id { get; set; }
            public string Name { get; set; }
            public int EmailId { get; set; }

            public Email Email { get; set; }
        }

        public void ManualQuerySample()
        {
            using (var ctx = new MyContext())
            {
                ctx.Database.EnsureDeleted();
                ctx.Database.EnsureCreated();

                var t11 = new Tag { Name = "t11" };
                var t12 = new Tag { Name = "t12" };
                var t13 = new Tag { Name = "t13" };
                var t21 = new Tag { Name = "t21" };
                var t22 = new Tag { Name = "t22" };
                var t31 = new Tag { Name = "t31" };

                var e1 = new Email { Subject = "email1", Tags = new List<Tag> { t11, t12, t13 } };
                var e2 = new Email { Subject = "email2", Tags = new List<Tag> { t21, t22 } };
                var e3 = new Email { Subject = "email3", Tags = new List<Tag> { t31 } };

                ctx.Tags.AddRange(t11, t12, t13, t21, t22, t31);
                ctx.Emails.AddRange(e1, e2, e3);
                ctx.SaveChanges();

                var emails = ctx.Emails.ToList();
            }

            using (var ctx = new MyContext())
            {
                var query = from e in ctx.Emails
                            where e.Id < 3
                            join t in ctx.Tags on e.Id equals t.EmailId into grouping
                            from t in grouping.Where(g => !g.Name.EndsWith("1")).DefaultIfEmpty()
                            select new { Email = e, t.Name };

                var result = query.ToList()
                        .GroupBy(key => key.Email, element => element.Name)
                        .Select(g => new EmailDto { Subject = g.Key.Subject, Tags = g.Select(t => new TagDto { Name = t }).ToList() });
            }
        }

It’s quite tricky to write, but you can do filtering and custom projections on inner and outer collections. Basically the idea is to create a groupjoin, which pairs up the email entities and their tags - you can apply filter on emails directly (see: where e.Id >3, and filters to tags are applied on the grouping, see from t in grouping.Where(g => !g.Name.EndsWith("1")).DefaultIfEmpty()

When you apply all the filters, you project both into anonymous type. This will produce the following SQL, that fetches all the (filtered) emails and tags in one query:

SELECT [e].[Id], [e].[Subject], [t0].[Name]
FROM [Emails] AS [e]
LEFT JOIN (
    SELECT [t].*
    FROM [Tags] AS [t]
    WHERE RIGHT([t].[Name], LEN(N'1')) <> N'1'
) AS [t0] ON [e].[Id] = [t0].[EmailId]
WHERE [e].[Id] < 3

now you need to group by the results by email, and shape the result into DTOs