How to fix the "Collection to which the Single Aggregate is applied should be empty or contain only one InvalidOperationException?

Question

Having this query, I get an InvalidOperationException: "A collection to which a single collection applies must be empty or contain exactly one element."

List<int> olsesUsedForTaskCompletion = new List<int>();
olsesUsedForTaskCompletion.AddRange(task.OrderLineSpecifications_QtysCompleted.Select(ols => ols.Key).ToList());

var allRelatedTasks = (from t in new XPQuery<Core.Model.Task.Task>(session, true)
                       join ols in new XPQuery<OrderLineSpecification>(session, true)
                       on t.PickSpecification equals ols.PickSpecification
                       where t.PickSpecification == task.PickSpecification
                          && t.Status != TaskStatuses.Cancelled
                          && olsesUsedForTaskCompletion.Contains(ols.Oid)
                       select t).ToList();

I want that when I make this connection, I get only OLS that have a specific Id. What am I doing wrong?

This is the stack trace:

at DevExpress.Xpo.Helpers.InTransactionLoader.ProcessException(Exception ex)
   at DevExpress.Xpo.Helpers.InTransactionLoader.ProcessAnalyzeAndExecQuery()
   at DevExpress.Xpo.Helpers.InTransactionLoader.Process()
   at DevExpress.Xpo.Helpers.InTransactionLoader.GetObjects(ObjectsQuery[] queries)
   at DevExpress.Xpo.Helpers.InTransactionLoader.GetObjects(Session session, ObjectsQuery[] queries)
   at DevExpress.Xpo.Session.<>c__DisplayClass16.<GetObjectsInTransaction>b__14()
   at DevExpress.Xpo.Logger.LogManager.Log[T](String category, LogHandler`1 handler, MessageHandler`1 createMessageHandler)
   at DevExpress.Xpo.Session.GetObjectsInTransaction(XPClassInfo classInfo, CriteriaOperator condition, SortingCollection sorting, Int32 skipSelectedRecords, Int32 topSelectedRecords, Boolean selectDeleted)
   at DevExpress.Xpo.XPQueryBase.SessionGetObjects(XPClassInfo classInfo, CriteriaOperator condition, SortingCollection sorting, Int32 skipSelectedRecords, Int32 topSelectedRecords, Boolean selectDeleted)
   at DevExpress.Xpo.XPQueryBase.GetObjects()
   at DevExpress.Xpo.XPQueryBase.Enumerate(Type type)
   at DevExpress.Xpo.XPQuery`1.GetEnumerator()
   at DevExpress.Xpo.XPQuery`1.System.Collections.Generic.IEnumerable<T>.GetEnumerator()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at Davanti.WMS.Services.Implementation.Outbound.OrderLineSpecificationStatusService.ChangeStatusToPickedToShipToStageOrStaged(Session session, IList`1 tasks, IList`1 olsWithoutTasks) in c:\Corax\DAV_WMS\DEV\SRC\APP\WMS\Davanti.WMS.Services.Implementation\Outbound\OrderLineSpecificationStatusService.cs:line 471

Update: After some battles, this is what I did: - a different approach came. I do not know if you can get the business logic, but I created the first list with OLS, and then from it I created another list with selection options. Later I make a simple query in Tasks.

// compose list of olses for which status will be updated
    List<OrderLineSpecification> olSpecs = (from ols in new XPQuery<OrderLineSpecification>(session, true)
                                            where ols.Status != OrderLineSpecificationStatus.Cancelled 
                                                    //...
                                                    && ols.PickSpecification == task.PickSpecification
                                                    && (olsesUsedForTaskCompletion.Count == 0
                                                        || (olsesUsedForTaskCompletion.Contains(ols.Oid) && ols.QtyOrdered == ols.QtyPicked))
                                            select ols).ToList();

    var pickSpecificationKeys = (from ols in olSpecs select ols.PickSpecification.Oid).Distinct().ToList();

    var allRelatedTasks = (from t in new XPQuery<Core.Model.Task.Task>(session, true)
                            where pickSpecificationKeys.Contains(t.PickSpecification.Oid)
                                    && t.Status != TaskStatuses.Cancelled
                            select t).ToList();

, , , duble ...:)

Answer 1

Select , , ( ). . , innerKey outerKey Join , .

, , . , "join in" :

from t in new XPQuery<Core.Model.Task.Task>(session, true)
   join ols in new XPQuery<OrderLineSpecification>(session, true)
   on t.PickSpecification equals ols.PickSpecification
   into tg
   where tg.key == task.PickSpecification
      && tg.Any(gi.Status != TaskStatuses.Cancelled && olsesUsedForTaskCompletion.Contains(ols.Oid))
   select t

, :

from ols in new XPQuery<OrderLineSpecification>(session, true)
   join t in new XPQuery<Core.Model.Task.Task>(session, true)
   on ols.PickSpecification equals t.PickSpecification
   where ols.PickSpecification == task.PickSpecification
      && t.Status != TaskStatuses.Cancelled
      && olsesUsedForTaskCompletion.Contains(ols.Oid)
   select t

, Select, .

from t in new XPQuery<Core.Model.Task.Task>(session, true)
   join ols in new XPQuery<OrderLineSpecification>(session, true)
   on t.PickSpecification equals ols.PickSpecification
   where t.PickSpecification == task.PickSpecification
      && t.Status != TaskStatuses.Cancelled
      && olsesUsedForTaskCompletion.Contains(ols.Oid)
   select new { SomeProperty = t.SomeProperty, AnotherProperty = t.AnotherProperty }

PS: , DevExpress . XPO, , , .