some progress

node_modules/mongoose/lib/helpers/populate/lookupLocalFields.js

@@ -0,0 +1,40 @@
+'use strict';
+
+module.exports = function lookupLocalFields(cur, path, val) {
+  if (cur == null) {
+    return cur;
+  }
+
+  if (cur._doc != null) {
+    cur = cur._doc;
+  }
+
+  if (arguments.length >= 3) {
+    if (typeof cur !== 'object') {
+      return void 0;
+    }
+    if (val === void 0) {
+      return void 0;
+    }
+    if (cur instanceof Map) {
+      cur.set(path, val);
+    } else {
+      cur[path] = val;
+    }
+    return val;
+  }
+
+
+  // Support populating paths under maps using `map.$*.subpath`
+  if (path === '$*') {
+    return cur instanceof Map ?
+      Array.from(cur.values()) :
+      Object.keys(cur).map(key => cur[key]);
+  }
+
+  if (cur instanceof Map) {
+    return cur.get(path);
+  }
+
+  return cur[path];
+};