diff --git a/CHANGELOG.md b/CHANGELOG.md
index bf42d0f..75c45c1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- **Mongo full-document Save (#778)** — JSON editor Save uses `replaceOne` (whole document, `_id` locked) instead of `$set`, so fields deleted in JSON — including nested keys — are removed on the server.
 - **Mongo document editor Back (#782)** — Dirty JSON in `MongoDocumentEditor` is registered with `UnsavedWorkRegistry`. Breadcrumb Back, Home, Close, and tree navigation confirm before discarding; Cancel keeps the editor.
 - **Mongo JSON filter ObjectId / DateTime (#783)** — Document list filter parses Extended JSON (`$oid`, `$date`) and wraps a 24-character hex `_id` as `ObjectId`. A leftover string `_id` with zero matches shows how to write `{ "_id": { "$oid": "…" } }`.
 - **Postgres open-transaction probe (#787)** — `BEGIN` + `SELECT` does not assign an XID, so `pg_current_xact_id_if_assigned` stayed NULL (SQL-tab badge off; autocommit-off prepended a second `BEGIN`). The session tracks BEGIN/COMMIT/ROLLBACK and otherwise probes `pg_stat_activity.xact_start` (PG 9+). Implicit BEGIN when autocommit is off stays a separate `execute`.
diff --git a/lib/core/database/mongodb_service.dart b/lib/core/database/mongodb_service.dart
index 4b09bab..7d4c59f 100644
--- a/lib/core/database/mongodb_service.dart
+++ b/lib/core/database/mongodb_service.dart
@@ -231,7 +231,7 @@ class MongoService {
     });
   }
 
-  /// Updates a single document matched by [filter].
+  /// Updates a single document matched by [filter] (`$set` / `$unset` / …).
   Future<void> updateDocument(
     MongoConnection connection,
     String database,
@@ -245,6 +245,20 @@ class MongoService {
     });
   }
 
+  /// Replaces a single document matched by [filter] (full-document Save).
+  Future<void> replaceDocument(
+    MongoConnection connection,
+    String database,
+    String collection,
+    Map<String, dynamic> filter,
+    Map<String, dynamic> replacement,
+  ) async {
+    return _withDb(connection, database, (db) async {
+      final coll = db.collection(collection);
+      await coll.replaceOne(filter, replacement);
+    });
+  }
+
   /// Deletes a single document matched by [filter].
   Future<void> deleteDocument(
     MongoConnection connection,
diff --git a/lib/features/mongodb/mongo_document_editor.dart b/lib/features/mongodb/mongo_document_editor.dart
index 6e6e52c..f183485 100644
--- a/lib/features/mongodb/mongo_document_editor.dart
+++ b/lib/features/mongodb/mongo_document_editor.dart
@@ -150,10 +150,12 @@ class _MongoDocumentEditorState extends material.State<MongoDocumentEditor> {
       return;
     }
 
-    // Remove _id from the update payload (can't change _id). Filter uses the
-    // original BSON _id, not a stringified copy from the editor.
-    final updateDoc = Map<String, dynamic>.from(parsed);
-    updateDoc.remove('_id');
+    // replaceOne drops keys the user deleted. `_id` stays the original BSON
+    // value even if the editor JSON changed it.
+    final replacement = mongoFullDocumentReplacement(
+      parsed: parsed,
+      originalId: id,
+    );
 
     setState(() {
       _saving = true;
@@ -162,12 +164,12 @@ class _MongoDocumentEditorState extends material.State<MongoDocumentEditor> {
     });
 
     try {
-      await MongoService.instance.updateDocument(
+      await MongoService.instance.replaceDocument(
         widget.connection,
         widget.database,
         widget.collection,
         {'_id': id},
-        {r'$set': updateDoc},
+        replacement,
       );
       if (!mounted) return;
       setState(() {
diff --git a/lib/features/mongodb/mongo_ejson.dart b/lib/features/mongodb/mongo_ejson.dart
index f20f002..d8afb8b 100644
--- a/lib/features/mongodb/mongo_ejson.dart
+++ b/lib/features/mongodb/mongo_ejson.dart
@@ -53,3 +53,17 @@ Map<String, dynamic> mongoFilterFromJson(String text) {
 bool mongoFilterNeedsObjectIdHint(Map<String, dynamic> filter) {
   return filter['_id'] is String;
 }
+
+/// Full-document Save payload for [DbCollection.replaceOne].
+///
+/// Parsed JSON minus any `_id` the user typed, then `_id` locked to
+/// [originalId]. Keys absent from [parsed] (including nested ones) are not in
+/// the replacement, so they are dropped — unlike `$set`, which only merges.
+Map<String, dynamic> mongoFullDocumentReplacement({
+  required Map<String, dynamic> parsed,
+  required Object originalId,
+}) {
+  final out = Map<String, dynamic>.from(parsed);
+  out['_id'] = originalId;
+  return out;
+}
diff --git a/test/features/mongodb/mongo_ejson_test.dart b/test/features/mongodb/mongo_ejson_test.dart
index 0b74c83..2bca49b 100644
--- a/test/features/mongodb/mongo_ejson_test.dart
+++ b/test/features/mongodb/mongo_ejson_test.dart
@@ -98,4 +98,39 @@ void main() {
       expect((filter['age'] as Map)[r'$gt'], 5);
     });
   });
+
+  group('mongoFullDocumentReplacement', () {
+    test('start {a:1,b:2}, save {a:1} drops b and keeps original _id', () {
+      final id = ObjectId.fromHexString('507f1f77bcf86cd799439011');
+      final replacement = mongoFullDocumentReplacement(
+        parsed: <String, dynamic>{'a': 1},
+        originalId: id,
+      );
+      expect(replacement.containsKey('b'), isFalse);
+      expect(replacement['a'], 1);
+      expect(replacement['_id'], same(id));
+    });
+
+    test('nested keys deleted in JSON are absent from the replacement', () {
+      final id = ObjectId.fromHexString('507f191e810c19729de860ea');
+      final replacement = mongoFullDocumentReplacement(
+        parsed: <String, dynamic>{
+          'nested': <String, dynamic>{'x': 1},
+        },
+        originalId: id,
+      );
+      expect(replacement['nested'], <String, dynamic>{'x': 1});
+      expect((replacement['nested'] as Map).containsKey('y'), isFalse);
+    });
+
+    test('edited _id in JSON is overwritten with the original BSON id', () {
+      final id = ObjectId.fromHexString('507f1f77bcf86cd799439011');
+      final replacement = mongoFullDocumentReplacement(
+        parsed: <String, dynamic>{'_id': 'tampered', 'a': 1},
+        originalId: id,
+      );
+      expect(replacement['_id'], same(id));
+      expect(replacement['a'], 1);
+    });
+  });
 }