How to add extra attribute in model to move calculation out of view?

Question

For the following RubyOnRails code, is there a way to move the "profit" calculation out of the View and into the Model.. so there's maybe an attribute called total_income and total_expense?

Model - transaction.rb

class Transaction < ActiveRecord::Base
  attr_accessible :name, :amount, :category
  scope :incomes,  :conditions => { :category => 'Income'  }
  scope :expenses, :conditions => { :category => 'Expense' }
end

Controller - transactions_controller.rb

class TransactionsController < ApplicationController

  def index
    @incomes  = Transaction.incomes
    @expenses = Transaction.expenses
    @transaction = Transaction.new
  end

View - index.html.erb

<pre>
<strong>Income</strong>
  <% @incomes.each do |income| %>
  <%= income.name %>  -  <%= number_to_currency((income.amount.nil? ? 0 : income.amount)) %>
  <% end %>
  <strong>Subtotal:</strong> <%= number_to_currency(@income_total = @incomes.sum(:amount)) %>

<strong>Expenses</strong>
  <% @expenses.each do |expense| %>
  <%= expense.name %>  -  <%= number_to_currency((expense.amount.nil? ? 0 : expense.amount)) %>
  <% end %>
  <strong>Subtotal:</strong> <%= number_to_currency(@expenses_total = @expenses.sum(:amount)) %>

<strong>Profit: <%= number_to_currency(@income_total - @expenses_total) %></strong>
</pre>

unrelated to your problem: watch out, Transaction is a reserved word in many programming environments and could lead to problems. I make it a habit to always tweak the name a bit to avoid problems. — DGM
– DGM, Commented Feb 25, 2012 at 1:35
@DGM thanks for pointing this out. Some RoR reserved words (which I've found does include 'transaction') are listed on the RoR old wiki. — Turgs
– Turgs, Commented Feb 25, 2012 at 6:14

xkickflip · Accepted Answer · 2012-02-25 01:32:54Z

2

For the most basic change you could just add class methods to Transaction

class Transaction < ActiveRecord::Base
  attr_accessible :name, :amount, :category
  scope :incomes,  :conditions => { :category => 'Income'  }
  scope :expenses, :conditions => { :category => 'Expense' }

  def self.income_total
    incomes.sum :amount
  end

  def self.expenses_total
    expenses.sum :amount
  end

end

class TransactionsController < ApplicationController

  def index
    @incomes  = Transaction.incomes
    @expenses = Transaction.expenses
    @income_total = Transaction.income_total
    @expenses_total = Transaction.expenses_total
    @transaction = Transaction.new
  end

answered Feb 25, 2012 at 1:32

xkickflip

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1 Comment

Turgs Over a year ago

Just the type of thing I was looking for. Thanks.

zetetic · Accepted Answer · 2012-02-25 01:04:47Z

1

These days I prefer to wrap these multiple, interdependent instance variables up into a new presenter class, something like this (untested):

class TransactionPresenter

  attr_reader :income_total, :expenses_total

  def initialize
    @incomes = Transaction.incomes
    @expenses = Transaction.expenses
  end

  def each_income(&block)
    @incomes.each(&block)
  end

  def each_expense(&block)
    @incomes.each(&block)
  end

  def income_total
    @income_total ||= number_to_currency(@incomes.sum(&:amount))
  end

  def expenses_total
    @expenses_total ||= number_to_currency(@expenses.sum(&:amount))
  end

  def name_and_amount(income_or_expense)
    "#{income_or_expense.name} - #{number_to_currency((income.amount.nil? ? 0 : income.amount))}"
  end

  def profit
    number_to_currency(income_total - expenses_total)
  end

end

# controller

def index
  @presenter = TransactionPresenter.new
end

# view

<pre>
<strong>Income</strong>
  <% @presenter.each_income do |income| %>
    <%= @presenter.name_and_amount %>
  <% end %>
  <strong>Subtotal:</strong> <%= @presenter.income_total %>

<strong>Expenses</strong>
  <% @presenter.each_expense do |expense| %>
    <%= @presenter.name_and_amount %>
  <% end %>
  <strong>Subtotal:</strong> <%= @presenter.expenses_total %>

<strong>Profit: <%= @presenter.profit %></strong>
</pre>

answered Feb 25, 2012 at 1:04

zetetic

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1 Comment

Turgs Over a year ago

I'll have to read up on the use of a presenter class. I've never seen this type of thing before.

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