python django fields and row calculation

Hi,

I am working on an leave management system and I am having a some difficulties trying to figure out how to accomplished this scenario.

for example i have a table of.

when a user apply for a leave i want to get the previous total_leave_balance of the applicant and minus his/her current num_days and assign the result to be the total_leave_balance. Please advise what would be the best way to approach this.

username	first_name	second_name	num_days	leave_entitlement	beginning_bal	Total_leave_balance
jtom	james	tom	3	12	23	20
tjohn	tom	john	2	2	5	7
atom	anna	tom	3	21	2
jtom	james	tom	3	4	2
atom	anna	tom	3	3	5

cheers,

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