Model code to module: wrong number of args, class/ instance method?

Question

I am trying to move some model code to a module.

The original model method:

I am trying to move some model code to a module.

The original model method:

class Book < ActiveRecord::Base
  def book_royalty(period='enddate', basis="Net receipts")
    #stuff
  end
end

So I add

include Calculation   

and move the method to a module:

module Calculation
  def book_royalty(period='enddate', basis="Net receipts")
    #stuff
  end
end

But now I'm getting

wrong number of arguments (2 for 0)

This is the error I also get if I make the method in the book.rb model a class method i.e. if I make the method name self.book_royalty(args).

Am I inadvertently making the methods moved to the module class methods? I'm using include in book.rb, not extend. How can I get the parent model to successfully include the module's methods?

Edit

book_royalty is called in the Royaltystatement model.

book.rb:

attr_accessor :book_royalty

royaltystatement.rb:

def initialize_arrays
  @royalty  = []
  ...
end

def sum_at_book_level(contract)
  contract.books.where(:exclude_from_royalty_calc => false).each do |book| 
    book.book_royalty("enddate", basis)
    @royalty.push(book.book_royalty("enddate", basis))
    # etc
end

Answer 1

Explanation:

Your module defines a method book_royalty that takes two arguments. Then, a couple of lines after the inclusion of that module you use class macro attr_accessor which defines two methods,

def book_royalty
  @book_royalty
end

def book_royalty= val
  @book_royalty = val
end

This effectively overwrites your book_royalty from the module. Now it accepts no arguments. Hence the error

wrong number of arguments (2 for 0)

when trying to execute line

book.book_royalty("enddate", basis)

You don't need attr_accessor or anything else in order to use a method from included module. It becomes available automatically.