Hack: Delete unmatched topics

While the `Import()` extension method will successfully remove unmatched topics from the topic graph—assuming e.g. `DeleteUnmatchedChildren` is enabled—it doesn't have access to the `ITopicRepository` in order to delete them. The `ITopicRepository` doesn't have a way to detect internal deletions either, however.

In the future, we should evaluate ways of baking this into `ITopicRepository` e.g. by temporarily moving deleted topics to a `Topic.DeletedChildren` collection. For now, however, we can hack around this by taking a snapshot of all topics in the graph before hand, finding the difference afterwards, and deleting those topics that are no longer present.

Technically, this will result in some topics being deleted which are already deleted (e.g., because their parent was already deleted), but, for better or for worse, the `ITopicRepository` handles that situation gracefully (i.e., without throwing an exception).

This is an ugly hack, and especially because it's not intuitive behavior when using the Data Transfer library. It doesn't require too much code, however, and works reliably, so it's an acceptable workaround until a more sophisticated solution can be accounted for in the `ITopicRepository`.

Author: JeremyCaney

OnTopic.Editor.AspNetCore/Controllers/EditorController.cs

@@ -4,6 +4,7 @@
 | Project       Topics Library
 \=============================================================================================================================*/
 using System;
+using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 using System.Text;
@@ -23,6 +24,7 @@
 using OnTopic.Internal.Diagnostics;
 using OnTopic.Mapping;
 using OnTopic.Metadata;
+using OnTopic.Querying;
 using OnTopic.Repositories;
 
 namespace OnTopic.Editor.AspNetCore.Controllers {
@@ -694,6 +696,11 @@ public async Task<IActionResult> Import(IFormFile jsonFile, [Bind(Prefix = "Impo
         return View(editorViewModel);
       }
 
+      /*------------------------------------------------------------------------------------------------------------------------
+      | INDEX TOPICS IN SCOPE
+      \-----------------------------------------------------------------------------------------------------------------------*/
+      var topics                = target.FindAll(t => t.Id >= 0).ToList();
+
       /*------------------------------------------------------------------------------------------------------------------------
       | IMPORT INTO TOPIC GRAPH
       >-------------------------------------------------------------------------------------------------------------------------
@@ -704,6 +711,20 @@ public async Task<IActionResult> Import(IFormFile jsonFile, [Bind(Prefix = "Impo
       target.Import(topicData, options);
       target.Import(topicData, options);
 
+      /*------------------------------------------------------------------------------------------------------------------------
+      | DELETE UNMATCHED TOPICS
+      >-------------------------------------------------------------------------------------------------------------------------
+      | ### HACK JJC20200327: The Data Transfer library doesnt have access to the ITopicRepository, so it can't delete topics.
+      | Instead, it removes them from the topic graph. But the ITopicRepository implementations don't have a means of detecting
+      | removed topics during a recursive save and, therefore, the deletions aren't persited to the database. To mitigate this,
+      | we evaluate the topic graph after the save, and then delete any orphans.
+      \-----------------------------------------------------------------------------------------------------------------------*/
+      var unmatchedTopics       = topics.Except(target.FindAll(t => t.Id >= 0));
+
+      foreach (var unmatchedTopic in unmatchedTopics) {
+        TopicRepository.Delete(unmatchedTopic);
+      }
+
       /*------------------------------------------------------------------------------------------------------------------------
       | SAVE
       >-------------------------------------------------------------------------------------------------------------------------