Can't process set operations after client evaluation efcore

Question

Ef Core receiving error

System.InvalidOperationException: Can't process set operations after client evaluation, consider moving the operation before the last Select() call (see issue #16243) at Microsoft.EntityFrameworkCore.Query.SqlExpressions.SelectExpression.ApplySetOperation(SetOperationType setOperationType, SelectExpression select2, Boolean distinct)

when executing

 public async Task<object> GetUnitsForDataTableAsync() =>
        
            await context.Units
             .Where(x => !x.TractUnitJunctions.Any())
              .Select(x => new
              {
                  x.Id,
                  x.UnitName,
                  x.UnitAcres,
                  TractNum = String.Empty,
                  Wells = String.Empty,
                  NumOfWells = 0,
              })
              .Union(
                        context.TractUnitJunctions
                        .Select(x => new
                         {
                            Id = x.UnitId,
                            x.Unit.UnitName,
                            x.Unit.UnitAcres,
                            x.Tract.TractNum,
                            Wells = string.Join(", ", x.TractUnitJunctionWellJunctions
                                              .Select(z => $"{z.Well.WellNum} ({z.Well.ApiNum})")
                                        ),
                            NumOfWells = x.TractUnitJunctionWellJunctions.Count()
                 }))
                .ToListAsync().ConfigureAwait(false);

however the function works fine if I break it up into two queries.

 public async Task<object> GetUnitsForDataTableAsync()
        {
            var List1 = await context.Units
             .Where(x => !x.TractUnitJunctions.Any())
              .Select(x => new
              {
                  x.Id,
                  x.UnitName,
                  x.UnitAcres,
                  TractNum = String.Empty,
                  Wells = String.Empty,
                  NumOfWells = 0,
              })
             .ToListAsync().ConfigureAwait(false);

            var List2 = await context.TractUnitJunctions
                 .Select(x => new
                 {
                     Id = x.UnitId,
                     x.Unit.UnitName,
                     x.Unit.UnitAcres,
                     x.Tract.TractNum,
                     Wells = string.Join(", ", x.TractUnitJunctionWellJunctions
                                              .Select(z => $"{z.Well.WellNum} ({z.Well.ApiNum})")
                                        ),
                     NumOfWells = x.TractUnitJunctionWellJunctions.Count()
                 })
                .ToListAsync().ConfigureAwait(false);


            return List1.Concat(List2);
        }

I've researched that error a bit but I'm unsure how to refactor the first query to get around that error

Answer 1

Accepted answer while it works is flawed in that you're eager loading the first list.

This is a known bug which you should be able to resolve by moving the Where clause after the Union

As discussed here https://github.com/dotnet/efcore/issues/16243#issuecomment-622452276