diff --git a/lib/ws.js b/lib/ws.js
index c38c031..352c59d 100644
--- a/lib/ws.js
+++ b/lib/ws.js
@@ -1,8 +1,22 @@
 import { WebSocketServer } from 'ws';
+import { ObjectId } from 'mongodb';
 
 let wss;
 let connections = [];
 
+function replaceIdInFilter (filter) {
+  if (filter) {
+    for (const key in filter) {
+      if (key == '_id' && typeof filter[key] == 'string') {
+        filter[key] = new ObjectId(filter[key]);
+      } else if (typeof filter[key] == 'object') {
+        filter[key] = replaceIdInFilter(filter[key]);
+      }
+    }
+  }
+  return filter;
+}
+
 export function init(cfg, db) {
   wss = new WebSocketServer(cfg);
 
@@ -21,7 +35,7 @@ export function init(cfg, db) {
           case "set":
             db.persist(
               message.subject,
-              message.filter,
+              replaceIdInFilter(message.filter),
               message.field,
               message.value
             );