budget/app/models/expense.rb

class Expense < ApplicationRecord
  PERIOD_OCCURENCES = {
    monthly: 12,
    annually: 1,
    weekly: 52,
    quarterly: 4,
  }.freeze
  enum :period, PERIOD_OCCURENCES.keys

  def monthly
    payment * PERIOD_OCCURENCES[period.to_sym] / 12
  end

  def self.total
    Expense.sum(&:payment)
  end

  def self.monthly_total
    Expense.all.map(&:monthly).sum.round(2)
  end

  def self.credit_card_monthly_total
    Expense.where(credit_card: true).map(&:monthly).sum.round(2)
  end
end