Union fails if one query has returned no results

Union fails if one query in the union returns no result. An InvalidOperationException is thrown with the message "When performing a set operation, both operands must have the same include operations. Investigation revealed that if one of the queries in the union is empty (returned no results) the union operation fails. In code example below any or the queries (1, 2, or 3) may return no results.

        public async Task<DelimitedList> AvailableClubsAsync(string clubMnemonic = null)
        {
            using var context = new PGSSqlServerContext();

            var query1 = context.Clubs.ForGolferAsync().Where(cl => cl.ClubMnemonic == clubMnemonic)
                                                                                   .Select(cl => new { cl.ClubMnemonic, Sequence = 0 });

            var query2 = context.Clubs.ForGolferAsync().Where(cl => !cl.Courses.Any())
                                                                                   .Select(cl => new { cl.ClubMnemonic, Sequence = 1 });

            var numberOfTees = await context.Tees.ForGolferAsync().CountAsync().ConfigureAwait(false);

            var query3 = context.Courses.ForGolferAsync().Where(co => co.ClubMnemonic != clubMnemonic)
                                                                                       .GroupBy(co => new { co.ClubMnemonic, co.CourseName })
                                                                                       .Where(g => (g.Count() < numberOfTees && g.Key.CourseName == null) || (g.Key.CourseName != null))
                                                                                       .Select(g => new { g.Key.ClubMnemonic, Sequence = 1 });

            var query = query1.Union(query2).Union(query3)
                                          .OrderBy(q => q.Sequence).ThenBy(q => q.ClubMnemonic)
                                          .Select(q => q.ClubMnemonic);

            return new DelimitedList(list: await query.ToListAsync());
        }

Further technical details

EF Core version: 3.1.1 Database provider: (e.g. Microsoft.EntityFrameworkCore.SqlServer) Target framework: (e.g. .NET Core 3.1) Operating system: Windows 10 IDE: (e.g. Visual Studio 2019 164…3)

[FormerMarine]

I tested a very small subset of a large data access layer. In addition to this problem I also encountered a problem with DefaultIfEmpty not accepting a default value. The performance of various queries was significantly worse than EF6.

I will definitely try again when we get a little closer to the 5.0 release date.

On Fri, Apr 24, 2020 at 7:04 AM Shay Rojansky [e-mail] wrote:

Sorry to hear that @FormerMarine https://github.com/FormerMarine. It would be good to know exactly which problems you ran into, and hopefully you’ll try again at some point.

— You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub https://github.com/dotnet/efcore/issues/19705#issuecomment-618995679, or unsubscribe https://github.com/notifications/unsubscribe-auth/ALZAM764NMA4ERD7H4TVRFTROGE5XANCNFSM4KLQPOLA .

[roji]

Note that this has already been fixed in 5.0.0-preview1 - you can give that a try as well. It would be great to get confirmation that the bug is gone.