code: function makeMongooseModelName(collectionName) {
      if (collectionName !== collectionName.toLowerCase()) {
          throw new Error('expect lowercase, was ' + collectionName);
      }
      if (collectionName.charAt(collectionName.length - 1) === 's') {
          return collectionName.substring(0, collectionName.length - 1);
      }
      throw new Error('expected name with trailing s');
  }
docs: return a modelname without a traling s
  @param collectionName