Adding Business Logic in ActiveRecord

not sure where you got the :condition from but try this

def accepted_accounttransactions
   accounttransactions.inject(0) {|total, at| total + (at.status ==
'accepted' ? at.amount : 0 }
end

try

def accepted_accounttransactions(filter)
  accounttransactions.inject(0) {|total, at| total + (at.status ==
filter ? at.amount : 0 }
end

model.accepted_accounttransactions('accepted')