some progress

node_modules/mongoose/lib/cast/string.js

@@ -0,0 +1,37 @@
+'use strict';
+
+const CastError = require('../error/cast');
+
+/**
+ * Given a value, cast it to a string, or throw a `CastError` if the value
+ * cannot be casted. `null` and `undefined` are considered valid.
+ *
+ * @param {Any} value
+ * @param {String} [path] optional the path to set on the CastError
+ * @return {string|null|undefined}
+ * @throws {CastError}
+ * @api private
+ */
+
+module.exports = function castString(value, path) {
+  // If null or undefined
+  if (value == null) {
+    return value;
+  }
+
+  // handle documents being passed
+  if (value._id && typeof value._id === 'string') {
+    return value._id;
+  }
+
+  // Re: gh-647 and gh-3030, we're ok with casting using `toString()`
+  // **unless** its the default Object.toString, because "[object Object]"
+  // doesn't really qualify as useful data
+  if (value.toString &&
+      value.toString !== Object.prototype.toString &&
+      !Array.isArray(value)) {
+    return value.toString();
+  }
+
+  throw new CastError('string', value, path);
+};