use a transaction when merging words to ensure changes are either all…

… made or not.

Backend.Tests/Controllers/MergeControllerTests.cs

@@ -14,6 +14,7 @@ public class MergeControllerTests : IDisposable
         private IMergeBlacklistRepository _mergeBlacklistRepo = null!;
         private IMergeGraylistRepository _mergeGraylistRepo = null!;
         private IWordRepository _wordRepo = null!;
+        private IMongoDbContext _mongoDbContext = null!;
         private IMergeService _mergeService = null!;
         private IPermissionService _permissionService = null!;
         private IWordService _wordService = null!;
@@ -42,7 +43,8 @@ public void Setup()
             _mergeGraylistRepo = new MergeGraylistRepositoryMock();
             _wordRepo = new WordRepositoryMock();
             _wordService = new WordService(_wordRepo);
-            _mergeService = new MergeService(_mergeBlacklistRepo, _mergeGraylistRepo, _wordRepo, _wordService);
+            _mongoDbContext = new MongoDbContextMock();
+            _mergeService = new MergeService(_mergeBlacklistRepo, _mergeGraylistRepo, _wordRepo, _wordService, _mongoDbContext);
             _permissionService = new PermissionServiceMock();
             _mergeController = new MergeController(_mergeService, _permissionService);
         }

Backend.Tests/Mocks/MongoDbContextMock.cs

@@ -0,0 +1,32 @@
+using System;
+using System.Threading.Tasks;
+using BackendFramework.Interfaces;
+using MongoDB.Driver;
+
+namespace Backend.Tests.Mocks;
+
+public class MongoDbContextMock: IMongoDbContext
+{
+    public IMongoDatabase Db => throw new NotSupportedException();
+    public Task<IMongoTransaction> BeginTransaction()
+    {
+        return Task.FromResult<IMongoTransaction>(new MongoTransactionMock());
+    }
+
+    private sealed class MongoTransactionMock : IMongoTransaction
+    {
+        public Task CommitTransactionAsync()
+        {
+            return Task.CompletedTask;
+        }
+
+        public Task AbortTransactionAsync()
+        {
+            return Task.CompletedTask;
+        }
+
+        public void Dispose()
+        {
+        }
+    }
+}

Backend.Tests/Services/MergeServiceTests.cs

@@ -14,6 +14,7 @@ public class MergeServiceTests
         private IMergeGraylistRepository _mergeGraylistRepo = null!;
         private IWordRepository _wordRepo = null!;
         private IWordService _wordService = null!;
+        private IMongoDbContext _mongoDbContext = null!;
         private IMergeService _mergeService = null!;
 
         private const string ProjId = "MergeServiceTestProjId";
@@ -26,7 +27,8 @@ public void Setup()
             _mergeGraylistRepo = new MergeGraylistRepositoryMock();
             _wordRepo = new WordRepositoryMock();
             _wordService = new WordService(_wordRepo);
-            _mergeService = new MergeService(_mergeBlacklistRepo, _mergeGraylistRepo, _wordRepo, _wordService);
+            _mongoDbContext = new MongoDbContextMock();
+            _mergeService = new MergeService(_mergeBlacklistRepo, _mergeGraylistRepo, _wordRepo, _wordService, _mongoDbContext);
         }
 
         [Test]

Backend/Services/MergeService.cs

@@ -15,14 +15,17 @@ public class MergeService : IMergeService
         private readonly IMergeGraylistRepository _mergeGraylistRepo;
         private readonly IWordRepository _wordRepo;
         private readonly IWordService _wordService;
+        private readonly IMongoDbContext _mongoDbContext;
 
         public MergeService(IMergeBlacklistRepository mergeBlacklistRepo, IMergeGraylistRepository mergeGraylistRepo,
-            IWordRepository wordRepo, IWordService wordService)
+            IWordRepository wordRepo, IWordService wordService,
+            IMongoDbContext mongoDbContext)
         {
             _mergeBlacklistRepo = mergeBlacklistRepo;
             _mergeGraylistRepo = mergeGraylistRepo;
             _wordRepo = wordRepo;
             _wordService = wordService;
+            _mongoDbContext = mongoDbContext;
         }
 
         /// <summary> Prepares a merge parent to be added to the database. </summary>
@@ -75,8 +78,11 @@ public async Task<List<Word>> Merge(string projectId, string userId, List<MergeW
         {
             var keptWords = mergeWordsList.Where(m => !m.DeleteOnly);
             var newWords = keptWords.Select(m => MergePrepParent(projectId, m).Result).ToList();
+            using var transaction = await _mongoDbContext.BeginTransaction();
             await Task.WhenAll(mergeWordsList.Select(m => MergeDeleteChildren(projectId, m)));
-            return await _wordService.Create(userId, newWords);
+            var words = await _wordService.Create(userId, newWords);
+            await transaction.CommitTransactionAsync();
+            return words;
         }
 
         /// <summary> Undo merge </summary>
@@ -92,15 +98,18 @@ public async Task<bool> UndoMerge(string projectId, string userId, MergeUndoIds
                 }
             }
 
+            using var transaction = await _mongoDbContext.BeginTransaction();
             // If children are not restorable, return without any undo.
             if (!await _wordService.RestoreFrontierWords(projectId, ids.ChildIds))
             {
+                // Nothing happens, transaction does not need to commit.
                 return false;
-            };
+            }
             foreach (var parentId in ids.ParentIds)
             {
                 await _wordService.DeleteFrontierWord(projectId, userId, parentId);
             }
+            await transaction.CommitTransactionAsync();
             return true;
         }