Devise\Pages\Collections\CollectionsRepository::findMissingFieldsForInstance PHP Method

    protected function findMissingFieldsForInstance($instance, $collectionFields)
    {
        $missingFields = [];
        foreach ($collectionFields as $collectionField) {
            $missing = true;
            $key = $collectionField['key'];
            foreach ($instance->fields as $field) {
                if ($field->key === $key) {
                    $missing = false;
                }
            }
            if ($missing) {
                $missingFields[$key] = $collectionField;
            }
        }
        return $missingFields;
    }