Negative Rollover option -- incorrect budget amount

[ScottAbbey]

When using the negative rollover option, it carries over the overspent amount in the available-to-budget calculation, but it should ignore that amount instead.

Example with current code:

January

Name	Amount
Funds for month	100
Overspent in prev month
Budgeted for month	-100
To be budgeted for month
Budgeted in the future

Category	Budgeted	Spent	Available
Food	50	-55	-5
Rent	50	-50

February

Name	Amount
Funds for month	100
Overspent in prev month	-5
Budgeted for month	-100
To be budgeted for month	-5
Budgeted in the future

Category	Budgeted	Spent	Available
Food	50	-50	-5
Rent	50	-50

Notice that the total amount available for the two months was 200, and 200 was budgeted, but the system says you have over-budgeted by 5.

Instead, it should look like this when negative rollover is used:

February

Name	Amount
Funds for month	100
Overspent in prev month
Budgeted for month	-100
To be budgeted for month
Budgeted in the future

Category	Budgeted	Spent	Available
Food	50	-50	-5
Rent	50	-50

Since the overspent amount from the previous month is left on the accounts as a negative available amount, we can't deduct that from the available amount to budget.

[isados]

heya, can you share your beancount file here.

[ScottAbbey]

@isados

Here is a short example file, I used the current release 0.5.9 to demonstrate the problem.

In the "2020-02" report, it shows the amount "to be budgeted" is zero and the amount "available" in the budget category is -15.00. Implying a total overspend of $15. But the actual overspend is only $10.

The $5 overspend from the first month has been 'doubled'. There should be $5 more "available" to budget. If you continued this pattern for a few months, it would just keep growing.

I did fix this trivially in my own fork, but the issue is still affecting anyone else who attempts to use the negative rollover option. In a larger beancount file, small overspends month after month could add up to many thousands eventually, without the user ever being aware.

option "title" "Beancount"
option "operating_currency" "USD"
2020-01-01 commodity USD

2000-01-01 custom "fava-extension" "fava_envelope" "{}"
2020-01-01 custom "envelope" "start date" "2020-01"
2020-01-01 custom "envelope" "negative rollover" "allow"
2020-01-01 custom "envelope" "budget account" "Assets:Checking"

2020-01-01 open Income:Work USD
2020-01-01 open Assets:Checking USD
2020-01-01 open Expenses:Food USD

2020-01-01 custom "envelope" "allocate" "Expenses:Food" 50.00
2020-01-01 * "Income"
  Assets:Checking                             50.00 USD
  Income:Work                                -50.00 USD
2020-01-01 * "Food"
  Assets:Checking                             -55.00 USD
  Expenses:Food                                55.00 USD

2020-02-01 custom "envelope" "allocate" "Expenses:Food" 45.00
2020-02-01 * "Income"
  Assets:Checking                              50.00 USD
  Income:Work                                 -50.00 USD
2020-02-01 * "Food"
  Assets:Checking                             -55.00 USD
  Expenses:Food                                55.00 USD

Generated report in Fava:

2020-01

Name	Amount
Funds for month	50.00
Overspent in prev month
Budgeted for month	-50.00
To be budgeted for month
Budgeted in the future

Account	Budgeted	Activity	Available
Expenses:Food	50.00	-55.00	-5.00

2020-02

Name	Amount
Funds for month	50.00
Overspent in prev month	-5.00
Budgeted for month	-45.00
To be budgeted for month
Budgeted in the future

Account	Budgeted	Activity	Available
Expenses:Food	45.00	-55.00	-15.00

[isados]

yup, you're absolutely right. got time now to check it out.
@polarmutex this is still an issue.