code: protected function mapField($field)
      {
          if (!$this->classMetadata->hasField($field)) {
              $this->classMetadata->mapField([
                  'fieldName' => $field,
                  'type'      => 'datetime',
                  'nullable'  => true,
              ]);
          }
      }
docs: Adds mapping to single field
  
  @param string $field