public function addRelation(Relation $relation) {
        $relation->setPrimaryModel($this);

        $this->_relations[$relation->getAlias()] = $relation;
        $this->_aliases[$relation->getRelatedClass()] = $relation->getAlias();
        $this->reserved[] = $relation->getAlias();

        $this->on('relation', $relation);

        return $this;
    }