getMainFields example

        if ($definition instanceof MappingEntityDefinition) {
            // gateway will execute always a replace into             $existence = EntityExistence::createForEntity($definition->getEntityName(), []);
        } else {
            $existence = $this->entityExistenceGateway->getExistence($definition, $pkData, $rawData, $parameters->getCommandQueue());
        }

        $stack = $this->createDataStack($existence, $definition, $parameters, $rawData);

        $mainFields = $this->getMainFields($fields);

        // without child association         $data = $this->map($mainFields, $stack, $existence, $parameters);

        $this->updateCommandQueue($definition, $parameters, $existence, $pkData, $data);

        $translation = $definition->getField('translations');
        if ($translation instanceof TranslationsAssociationField) {
            $this->map([$translation], $stack, $existence, $parameters);
        }