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SelectMany subquery not applying OrderBy

See original GitHub issue

Applying an OrderBy to a subquery in a SelectMany statement does not appear to work unless either a skip or take call is applied as well.

Database Structure

public class AppDbContext : DbContext
{
	public AppDbContext(DbContextOptions<AppDbContext> options) : base(options)
	{
	}

	public DbSet<Thread> Threads { get; set; }
	public DbSet<Post> Posts { get; set; }
}

public class Thread
{
	public Guid Id { get; set; }
	public ICollection<Post> Posts { get; set; }
}

public class Post 
{
	public Guid Id { get; set; }
	public string Message { get; set; }
	public Guid ThreadId { get; set; }
	public Thread Thread { get; set; }
}

Query

This query will produce the following SQL.

using var dbContext = new AppDbContext(dbOptions.Options);
var query = dbContext.Posts;
var data = query
	.Select(x => x.ThreadId)
	.Distinct()
	.SelectMany(x => query
		.Where(y => y.ThreadId == x)
		.OrderBy(y => y.Message))
	.ToList();

Will make

SELECT "t0"."Id", "t0"."Message", "t0"."ThreadId"
FROM (
	SELECT DISTINCT "p"."ThreadId"
	FROM "Posts" AS "p"
) AS "t"
INNER JOIN (
	SELECT "p0"."Id", "p0"."Message", "p0"."ThreadId"
	FROM "Posts" AS "p0"
) AS "t0" ON "t"."ThreadId" = "t0"."ThreadId"

You can see that there is no ORDER BY added to the generated SQL, However if I add a .Skip(0) to the subquery then an order by is generated in the sql.

using var dbContext = new AppDbContext(dbOptions.Options);
var query = dbContext.Posts;
var data = query
	.Select(x => x.ThreadId)
	.Distinct()
	.SelectMany(x => query
		.Where(y => y.ThreadId == x)
		.OrderBy(y => y.Message)
		.Skip(0))
	.ToList();

Will make

SELECT "t1"."Id", "t1"."Message", "t1"."ThreadId"
FROM (
	SELECT DISTINCT "p"."ThreadId"
	FROM "Posts" AS "p"
) AS "t"
INNER JOIN (
	SELECT "t0"."Id", "t0"."Message", "t0"."ThreadId"
	FROM (
		SELECT "p0"."Id", "p0"."Message", "p0"."ThreadId", ROW_NUMBER() OVER(PARTITION BY "p0"."ThreadId" ORDER BY "p0"."Message") AS "row"
		FROM "Posts" AS "p0"
	) AS "t0"
	WHERE 0 < "t0"."row"
) AS "t1" ON "t"."ThreadId" = "t1"."ThreadId"

We can see that now the order by over the partitions is working correctly. I have also observed that if there is a Take on the subquery instead of a Skip then it will also add the order by to the sql correctly as well.

Version Info

EF Core version: Database provider: Have observed in both Microsoft.EntityFrameworkCore.Sqlite 5.0.5 and Npgsql.EntityFrameworkCore.PostgreSQL 5.0.2 Target framework: .NET 5.0 Operating system: Windows 10, Ubuntu 20.04 IDE: Rider, VS Code

Full example project here: https://github.com/jack775544/EfOrder