Include sharding key if it is available in the document

mongo-migration-stream-core/src/main/kotlin/pl/allegro/tech/mongomigrationstream/core/synchronization/ChangeEvent.kt

@@ -81,23 +81,37 @@ internal data class InsertReplaceChangeEvent(
 
         fun fromMongoChangeStreamDocument(
             changeStreamDocument: ChangeStreamDocument<BsonDocument>,
-            shardingKey: String?
-        ): InsertReplaceChangeEvent =
-            InsertReplaceChangeEvent(
-                changeStreamDocument.operationType,
-                upsertKey(changeStreamDocument.documentKey!!, shardingKey),
+            shardingKey: String?,
+        ): InsertReplaceChangeEvent {
+            val documentFromEvent =
                 changeStreamDocument.fullDocument?.toBsonDocument(
                     BsonDocument::class.java,
-                    codecRegistry
+                    codecRegistry,
                 )
+
+            return InsertReplaceChangeEvent(
+                changeStreamDocument.operationType,
+                upsertKey(changeStreamDocument.documentKey!!, shardingKey, documentFromEvent),
+                documentFromEvent,
             )
+        }
 
         // This addresses "Failed to target upsert by query :: could not extract exact shard key" error for sharded destination collections
         // https://www.mongodb.com/docs/manual/reference/method/db.collection.update/#sharded-collections
-        private fun upsertKey(documentKey: BsonDocument, shardingKey: String?): BsonDocument {
-            if (shardingKey == null) return documentKey // When no shardingKey, don't change documentKey
-            if (documentKey.containsKey(shardingKey)) return documentKey // When sharding key is already in documentKey, don't change documentKey
-            return documentKey.append(shardingKey, BsonNull.VALUE) // When there is no shardingKey, add "shardingKey: null" to documentKey
+        private fun upsertKey(
+            documentKey: BsonDocument,
+            shardingKey: String?,
+            documentFromEvent: BsonDocument?,
+        ): BsonDocument {
+            // When no shardingKey, don't change documentKey
+            if (shardingKey == null) return documentKey
+            // When sharding key is already in documentKey, don't change documentKey
+            if (documentKey.containsKey(shardingKey)) return documentKey
+            // When destination collection is sharded, and there is no shardingKey, add "shardingKey: <value>" to documentKey
+            return documentKey.append(
+                shardingKey,
+                documentFromEvent?.get(shardingKey, BsonNull.VALUE) ?: BsonNull.VALUE,
+            )
         }
     }