Handle legacy attempts during statistics backfill

api/QuestionsApi/Services/AnswerStatisticsStore.cs

@@ -154,6 +154,10 @@ public sealed class AnswerStatisticsStore
         {
             return false;
         }
+        catch (MongoCommandException exception) when (exception.Code == 11000)
+        {
+            return false;
+        }
     }
 
     public async Task MarkBackfillCompleteAsync(string owner)
@@ -203,7 +207,13 @@ public sealed class AnswerStatisticsStore
             new BsonDocument("$match", match),
             new BsonDocument("$group", new BsonDocument
             {
-                { "_id", new BsonDocument { { "q", "$QuestionExternalId" }, { "b", "$CounterBucket" }, { "c", "$ChoiceId" } } },
+                { "_id", new BsonDocument
+                    {
+                        { "q", "$QuestionExternalId" },
+                        { "b", new BsonDocument("$ifNull", new BsonArray { "$CounterBucket", 0 }) },
+                        { "c", "$ChoiceId" }
+                    }
+                },
                 { "count", new BsonDocument("$sum", 1) }
             })
         };
@@ -214,7 +224,13 @@ public sealed class AnswerStatisticsStore
             new BsonDocument("$unwind", "$PresentedChoiceIds"),
             new BsonDocument("$group", new BsonDocument
             {
-                { "_id", new BsonDocument { { "q", "$QuestionExternalId" }, { "b", "$CounterBucket" }, { "c", "$PresentedChoiceIds" } } },
+                { "_id", new BsonDocument
+                    {
+                        { "q", "$QuestionExternalId" },
+                        { "b", new BsonDocument("$ifNull", new BsonArray { "$CounterBucket", 0 }) },
+                        { "c", "$PresentedChoiceIds" }
+                    }
+                },
                 { "count", new BsonDocument("$sum", 1) }
             })
         };
@@ -231,7 +247,12 @@ public sealed class AnswerStatisticsStore
             new BsonDocument("$match", legacyMatch),
             new BsonDocument("$group", new BsonDocument
             {
-                { "_id", new BsonDocument { { "q", "$QuestionExternalId" }, { "b", "$CounterBucket" } } },
+                { "_id", new BsonDocument
+                    {
+                        { "q", "$QuestionExternalId" },
+                        { "b", new BsonDocument("$ifNull", new BsonArray { "$CounterBucket", 0 }) }
+                    }
+                },
                 { "count", new BsonDocument("$sum", 1) }
             })
         };