class DatabaseFileUsageBackend

class DatabaseFileUsageBackend extends FileUsageBase {
    
    /**
     * The database connection used to store file usage information.
     *
     * @var \Drupal\Core\Database\Connection
     */
    protected $connection;
    
    /**
     * The name of the SQL table used to store file usage information.
     *
     * @var string
     */
    protected $tableName;
    
    /**
     * Construct the DatabaseFileUsageBackend.
     *
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
     *   The config factory.
     * @param \Drupal\Core\Database\Connection $connection
     *   The database connection which will be used to store the file usage
     *   information.
     * @param string $table
     *   (optional) The table to store file usage info. Defaults to 'file_usage'.
     */
    public function __construct(ConfigFactoryInterface $config_factory, Connection $connection, $table = 'file_usage') {
        parent::__construct($config_factory);
        $this->connection = $connection;
        $this->tableName = $table;
    }
    
    /**
     * {@inheritdoc}
     */
    public function add(FileInterface $file, $module, $type, $id, $count = 1) {
        $this->connection
            ->merge($this->tableName)
            ->keys([
            'fid' => $file->id(),
            'module' => $module,
            'type' => $type,
            'id' => $id,
        ])
            ->fields([
            'count' => $count,
        ])
            ->expression('count', '[count] + :count', [
            ':count' => $count,
        ])
            ->execute();
        parent::add($file, $module, $type, $id, $count);
    }
    
    /**
     * {@inheritdoc}
     */
    public function delete(FileInterface $file, $module, $type = NULL, $id = NULL, $count = 1) {
        // Delete rows that have an exact or less value to prevent empty rows.
        $query = $this->connection
            ->delete($this->tableName)
            ->condition('module', $module)
            ->condition('fid', $file->id());
        if ($type && $id) {
            $query->condition('type', $type)
                ->condition('id', $id);
        }
        if ($count) {
            $query->condition('count', $count, '<=');
        }
        $result = $query->execute();
        // If the row has more than the specified count decrement it by that number.
        if (!$result && $count > 0) {
            $query = $this->connection
                ->update($this->tableName)
                ->condition('module', $module)
                ->condition('fid', $file->id());
            if ($type && $id) {
                $query->condition('type', $type)
                    ->condition('id', $id);
            }
            $query->expression('count', '[count] - :count', [
                ':count' => $count,
            ]);
            $query->execute();
        }
        parent::delete($file, $module, $type, $id, $count);
    }
    
    /**
     * {@inheritdoc}
     */
    public function listUsage(FileInterface $file) {
        $result = $this->connection
            ->select($this->tableName, 'f')
            ->fields('f', [
            'module',
            'type',
            'id',
            'count',
        ])
            ->condition('fid', $file->id())
            ->condition('count', 0, '>')
            ->execute();
        $references = [];
        foreach ($result as $usage) {
            $references[$usage->module][$usage->type][$usage->id] = $usage->count;
        }
        return $references;
    }

}