Django rest framework cast foreign mysql foreign table column

пожалуйста, помогите мне преобразовать cast foreign table(customers) first name

class LoandetailListSearch(generics.ListAPIView):
  serializer_class = LoandetailSerializer

    def get_queryset(self):
        """
        Optionally restricts the returned loan details to a search list,
        by filtering against a `fields` in query parameter in the URL.
        """
        queryset = Loandetails.objects.all().exclude(broker_name__isnull=True).\
            extra(
                    {                    
                        'staff': "CONVERT(CAST(CONVERT(CONCAT(staff ) using latin1) as binary) using utf8)", 
                        'customer__first_name': 'SELECT CONVERT(CAST(CONVERT(CONCAT(first_name ) using latin1) as binary) using utf8) as first_name FROM customers WHERE customers.id = loan_details.customer_id'

                    }).\
            extra(select={'customer__first_name': 'SELECT CONVERT(CAST(CONVERT(CONCAT(first_name ) using latin1) as binary) using utf8) as first_name FROM customers WHERE customers.id = loan_details.customer_id' })

        return queryset

Я пытался преобразовать и привести оба метода first_name, но безуспешно. Спасибо

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