serializers.py

class LogEntryDetailsContractActivationSerialiser(serializers.ModelSerializer):
    number = serializers.CharField(source='number.number')
    name = serializers.CharField(source='contract.name')

    class Meta:
        model = UserContract
        fields = (
            'number',
            'name'
        )


class LogEntryContractActivationSerializer(serializers.ModelSerializer):
    user = serializers.CharField(source='user.username')
    details = serializers.SerializerMethodField()
    operation_type = serializers.SerializerMethodField()
    account = serializers.SerializerMethodField()
    amount = serializers.SerializerMethodField()
    balance = serializers.CharField(source='user.euro.balance.amount')

    class Meta:
        model = UserContract
        fields = (
            'user',
            'operation_type',
            'account',
            'amount',
            'balance',
            'details',
            'created_at',
        )

    def get_details(self, obj):
        return LogEntryDetailsContractActivationSerialiser(obj).data

    def get_operation_type(self, obj):
        return LogEntry.CONTRACT_ACTIVATION

    def get_account(self, obj):
        return LogEntry.EURO

    def get_amount(self, obj):
        return (-1) * obj.activation_amount.amount

views.py

    def update(self, request, *args, **kwargs):
        user = request.user
        serializer = self.get_serializer(data=request.data)
        serializer.is_valid(raise_exception=True)

        with cache.lock('buy_contract_{}'.format(user.id)):
            contract = Contract.objects.get(pk=serializer.data['contract_type_id'])
            if user.euro.balance < contract.activation_amount:
                raise ValidationError({'detail': _('insufficient funds')})
            if UserContract.objects.filter(user=user, contract=contract, finished_at=None).exists():
                raise ValidationError({'detail': _('you already have {} contract').format(contract.name)})

            user.save()

            user_contract = UserContract.objects.create(
                user=user,
                contract=contract,
                activation_amount=contract.activation_amount,
                balance=contract.activation_amount
            )

            Transaction.objects.create(
                source=user.euro,
                target=SystemCashAccount.objects.get(slug='contract'),
                amount=contract.activation_amount,
                tx_type=TransactionType.objects.get(slug='contract_activation'),
                comment="UserContract#{}".format(user_contract.pk)
            )
            user.euro.refresh_from_db()

            serializer = LogEntryContractActivationSerializer(user_contract)
            LogEntry.objects.create(serializer.data)

            # LogEntry.objects.create(
            #     user=user,
            #     operation_type=LogEntry.CONTRACT_ACTIVATION,
            #     account=LogEntry.EURO,
            #     amount=(-1) * contract.activation_amount.amount,
            #     balance=user.euro.balance.amount,
            #     details={
            #         'number': user_contract.number.number,
            #         'name': user_contract.contract.name
            #     }
            # )