serialize methods by to_json

I don't believe you can pass params using the methods option. One
approach would be to calculate the outcome on the collection before
calling to_json.

def calculate_outcome(arg)
@outcome = # do calculation with arg
end

def outcome
  @outcome
end

collection.each {|c| c.calculate_outcome(arg)}
collection.to_json(:methods => [:income, :outcome])

HTH,
Nicholas