some progress

node_modules/mongoose/lib/helpers/schema/idGetter.js

@@ -0,0 +1,32 @@
+'use strict';
+
+/*!
+ * ignore
+ */
+
+module.exports = function addIdGetter(schema) {
+  // ensure the documents receive an id getter unless disabled
+  const autoIdGetter = !schema.paths['id'] &&
+    schema.paths['_id'] &&
+    schema.options.id;
+  if (!autoIdGetter) {
+    return schema;
+  }
+
+  schema.virtual('id').get(idGetter);
+
+  return schema;
+};
+
+/**
+ * Returns this documents _id cast to a string.
+ * @api private
+ */
+
+function idGetter() {
+  if (this._id != null) {
+    return String(this._id);
+  }
+
+  return null;
+}