EventListener/TimestampableSubscriber.php
protected function mapField($field)
    {
        if (!$this->classMetadata->hasField($field)) {
            $this->classMetadata->mapField([
                'fieldName' => $field,
                'type'      => 'datetime',
                'nullable'  => true,
            ]);
        }
    }
protected function mapField($field)
    {
        if (!$this->classMetadata->hasField($field)) {
            $this->classMetadata->mapField([
                'fieldName' => $field,
                'type'      => 'datetime',
                'nullable'  => true,
            ]);
        }
    }
Adds mapping to single field

@param string $field