code: function(sql, attrs, callback) {
      var promise = this.definition.store.connection.raw(sql, attrs)
  
      if (typeof callback === 'function') {
        promise.then(callback)
      }
  
      return promise
    }
docs: execute raw sql
  @class Model
  @method raw
  @param {string} sql - The raw sql query.
  @param {array} attrs - Query attributes.
  @param {function} callback - The callback.
  
  @see Model.exec
  
  @return {Model}