private function defineManyToManyRelation(Model $model1, Model $model2) {
        //$this->app['log']->info(sprintf('%s has and belongs to many %s', $model1->getName(), str_plural($model2->getName())));
        $this->relations[$model1->getName()][] = new Relation($model1, $model2, Relation::HAS_AND_BELONGS_TO);
        $this->relations[$model2->getName()][] = new Relation($model2, $model1, Relation::HAS_AND_BELONGS_TO);
    }