protected function getExpectedIdentifiers($context = null)
    {
        if ($context === null) {
            throw new Exception\RuntimeException(
                'Expected context to be an array but is null'
            );
        }

        $className = $this->objectRepository->getClassName();

        if ($context instanceof $className) {
            return $this->objectManager
                        ->getClassMetadata($this->objectRepository->getClassName())
                        ->getIdentifierValues($context);
        }

        $result = [];
        foreach ($this->getIdentifiers() as $identifierField) {
            if (! array_key_exists($identifierField, $context)) {
                throw new Exception\RuntimeException(\sprintf('Expected context to contain %s', $identifierField));
            }

            $result[$identifierField] = $context[$identifierField];
        }
        return $result;
    }