yii\elasticsearch\ActiveDataProvider::prepareKeys PHP Method

    protected function prepareKeys($models)
    {
        $keys = [];
        if ($this->key !== null) {
            foreach ($models as $model) {
                if (is_string($this->key)) {
                    $keys[] = $model[$this->key];
                } else {
                    $keys[] = call_user_func($this->key, $model);
                }
            }
            return $keys;
        } elseif ($this->query instanceof ActiveQueryInterface) {
            /* @var $class \yii\db\ActiveRecord */
            $class = $this->query->modelClass;
            $pks = $class::primaryKey();
            if (count($pks) === 1) {
                foreach ($models as $model) {
                    $keys[] = $model->primaryKey;
                }
            } else {
                foreach ($models as $model) {
                    $kk = [];
                    foreach ($pks as $pk) {
                        $kk[$pk] = $model[$pk];
                    }
                    $keys[] = $kk;
                }
            }
            return $keys;
        } else {
            return array_keys($models);
        }
    }