Fix reference synchronization issue

Author: alex-kulakov

Orm/Xtensive.Orm.Tests/Issues/IssueJira0792_UnableToRemoveAssignedEntityWithNonNullableAssociationField.cs

@@ -0,0 +1,87 @@
+// Copyright (C) 2020 Xtensive LLC.
+// All rights reserved.
+// For conditions of distribution and use, see license.
+// Created by: Alexey Kulakov
+// Created:    2020.02.14
+
+using System;
+using System.Collections.Generic;
+using System.Text;
+using NUnit.Framework;
+using Xtensive.Orm.Configuration;
+using Xtensive.Orm.Validation;
+using Xtensive.Orm.Tests.Issues.IssueJira0792_UnableToRemoveAssignedEntityWithNonNullableAssociationFieldModel;
+
+namespace Xtensive.Orm.Tests.Issues.IssueJira0792_UnableToRemoveAssignedEntityWithNonNullableAssociationFieldModel
+{
+  [HierarchyRoot]
+  public class Job : Entity
+  {
+    [Field, Key]
+    public long Id { get; private set; }
+
+    [Field]
+    [Association(PairTo = nameof (JobTechnology.ReqiredJob), OnTargetRemove = OnRemoveAction.Clear, OnOwnerRemove = OnRemoveAction.Cascade)]
+    public JobTechnology Technology { get; set; }
+
+    public Job(Session session)
+      : base(session)
+    {
+    }
+  }
+
+  [HierarchyRoot]
+  public class JobTechnology : Entity
+  {
+    [Field, Key]
+    public long Id { get; set; }
+
+    [Field(Nullable = false), NotNullConstraint]
+    public Job ReqiredJob { get; set; }
+
+    public JobTechnology(Session session, Job job)
+      : base(session)
+    {
+      ReqiredJob = job;
+    }
+  }
+}
+
+namespace Xtensive.Orm.Tests.Issues
+{
+  public class IssueJira0792_UnableToRemoveAssignedEntityWithNonNullableAssociationField : AutoBuildTest
+  {
+    protected override DomainConfiguration BuildConfiguration()
+    {
+      var configuration = base.BuildConfiguration();
+      configuration.Types.Register(typeof (Job).Assembly, typeof (Job).Namespace);
+      configuration.UpgradeMode = DomainUpgradeMode.Recreate;
+      return configuration;
+    }
+
+    protected override void PopulateData()
+    {
+      using (var session = Domain.OpenSession())
+      using (var transaction = session.OpenTransaction()) {
+        new JobTechnology(session, new Job(session));
+        new JobTechnology(session, new Job(session));
+        new JobTechnology(session, new Job(session));
+        new JobTechnology(session, new Job(session));
+
+        transaction.Complete();
+      }
+    }
+
+    [Test]
+    public void MainTest()
+    {
+      using (var session = Domain.OpenSession())
+      using (var transaction = session.OpenTransaction()) {
+        foreach (var job in session.Query.All<Job>()) {
+          job.Technology.Remove();
+          session.SaveChanges();
+        }
+      }
+    }
+  }
+}

Orm/Xtensive.Orm.Tests/Xtensive.Orm.Tests.csproj

@@ -348,6 +348,7 @@
     <Compile Include="Issues\IssueJira0778_PrefetchStackOverflow.cs" />
     <Compile Include="Issues\IssueJira0735_WrongTypeIdsInValidateMode.cs" />
     <Compile Include="Issues\IssueJira0779_SetOperationsWithOrderAndTakeWrongQuery.cs" />
+    <Compile Include="Issues\IssueJira0792_UnableToRemoveAssignedEntityWithNonNullableAssociationField.cs" />
     <Compile Include="Issues\IssueJira_0530_LowSpeedDecimalMaterializationCouseSqlDecimalInternalException.cs" />
     <Compile Include="Issues\Issue_0694_SchemaUpgradeBug\ModelVersion1.cs" />
     <Compile Include="Issues\Issue_0694_SchemaUpgradeBug\ModelVersion2.cs" />

Orm/Xtensive.Orm/Orm/Internals/ReferentialActions.cs

@@ -36,7 +36,7 @@ private static void OnClearReference(AssociationInfo association, IEntity owner,
     {
       var nullIsEntity = owner as IHasNullEntity;
       var nullValue = nullIsEntity==null ? null : nullIsEntity.NullEntity;
-      if (nullValue!=null || association.OwnerField.IsNullable)
+      if (nullValue!=null || association.OwnerField.IsNullable || removalContext.Contains((Entity) owner))
         // If field is non-nullable & null value is real null, we should avoid assigning it,
         // since this will lead to an error on persist in almost any case;
         // but if we won't assign it, it will either fail with ref. constraint violation later,