code: function (id, field, values) {
              var self = this;
  
              var query = { _id: waterline.nodes.mongo.objectId(id)};
              query[field] = {$exists : false};
  
              var update = { $set: {} };
              update.$set[field] = values;
  
              var options = {new: true};
              return self.findAndModifyMongo(query, {}, update, options);
          }
docs: Using an id, find document and update fields only if they don't exist.
  Returns a waterline (bluebird) promise with the document find and modify.
  
  @param {string} id
  @param {String} field
  @param {Object/String/Number etc.} values
  @returns {Promise}