some progress

node_modules/mongoose/lib/helpers/query/selectPopulatedFields.js

@@ -0,0 +1,49 @@
+'use strict';
+
+const isExclusive = require('../projection/isExclusive');
+const isInclusive = require('../projection/isInclusive');
+
+/*!
+ * ignore
+ */
+
+module.exports = function selectPopulatedFields(fields, userProvidedFields, populateOptions) {
+  if (populateOptions == null) {
+    return;
+  }
+
+  const paths = Object.keys(populateOptions);
+  userProvidedFields = userProvidedFields || {};
+  if (isInclusive(fields)) {
+    for (const path of paths) {
+      if (!isPathInFields(userProvidedFields, path)) {
+        fields[path] = 1;
+      } else if (userProvidedFields[path] === 0) {
+        delete fields[path];
+      }
+    }
+  } else if (isExclusive(fields)) {
+    for (const path of paths) {
+      if (userProvidedFields[path] == null) {
+        delete fields[path];
+      }
+    }
+  }
+};
+
+/*!
+ * ignore
+ */
+
+function isPathInFields(userProvidedFields, path) {
+  const pieces = path.split('.');
+  const len = pieces.length;
+  let cur = pieces[0];
+  for (let i = 1; i < len; ++i) {
+    if (userProvidedFields[cur] != null || userProvidedFields[cur + '.$'] != null) {
+      return true;
+    }
+    cur += '.' + pieces[i];
+  }
+  return userProvidedFields[cur] != null || userProvidedFields[cur + '.$'] != null;
+}