Computed Column specification based on child table attributes in web2py

[rahulserver]

I have 2 tables:
db.define_table('Transaction_Master',Field('Account',db.Account_Master,requires=IS_IN_DB(db,'Account_Master.id', '%(Account)s %(State)s',zero=T('choose one'))),Field('Exam_Date','date'),Field('Entry_Date','date',default=request.now),Field('Form_1','upload'),Field('Form_1_Name'),
Field('Schoolwise_Form','upload'),Field('Schoolwise_Form_Name'),format='%(Account)s %(Exam_Date)s')

and:
db.define_table('Transactions',Field('TID',db.Transaction_Master),Field('School'),Field('Number_of_students','integer'))

The db.Transactions table will be populated by the excel sheet in Transaction_Master.Schoolwise_Form which contains the list of school names and number of students. I wish to add a computed column in the first table(virtual) which will simply sum up the number of students in Transactions referenced by Transaction_Master.tid. How do i do it?