use short-form presence validator.

use two-argument form.
add unpaid scope.
repair tests.

app/controllers/conference_registrations_controller.rb

@@ -28,7 +28,7 @@ class ConferenceRegistrationsController < ApplicationController
   end
 
   def show
-    @total_price = Ticket.total_price(@conference, current_user, :paid => true)
+    @total_price = Ticket.total_price(@conference, current_user, paid: true)
     @tickets = current_user.ticket_purchases.where(conference_id: @conference.id, paid: true)
     @ticket_payments = @tickets.group_by(&:payment_id)
   end

app/controllers/ticket_purchases_controller.rb

@@ -7,7 +7,7 @@ class TicketPurchasesController < ApplicationController
     TicketPurchase.destroy_all(user_id: current_user.id, conference_id: @conference.id, paid: false)
     message = TicketPurchase.purchase(@conference, current_user, params[:tickets][0])
     if message.blank?
-      if current_user.ticket_purchases.where(paid: false).any?
+      if current_user.ticket_purchases.unpaid.any?
         redirect_to new_conference_payment_path, notice: 'Please pay here to purchase tickets.'
       else
         redirect_to conference_conference_registration_path(@conference.short_title)

app/models/ticket.rb

@@ -21,8 +21,8 @@ class Ticket < ActiveRecord::Base
     ticket_purchases.find_by(user: user, paid: true).present?
   end
 
-  def quantity_bought_by(user, hashed_paid)
-   purchased_tickets = ticket_purchases.where(user_id: user.id, paid: hashed_paid[:paid])
+  def quantity_bought_by(user, paid: false)
+    purchased_tickets = ticket_purchases.where(user_id: user.id, paid: paid)
     quantity = 0
     if purchased_tickets
       purchased_tickets.each do |ticket|
@@ -36,16 +36,16 @@ class Ticket < ActiveRecord::Base
     ticket_purchases.find_by(user: user, paid: false).present?
   end
 
-  def total_price(user, hashed_paid)
-    quantity_bought_by(user, hashed_paid) * price
+  def total_price(user, paid: false)
+    quantity_bought_by(user, paid: paid) * price
   end
 
-  def self.total_price(conference, user, hashed_paid)
+  def self.total_price(conference, user, paid: false)
     tickets = Ticket.where(conference_id: conference.id)
     result = nil
     begin
       tickets.each do |ticket|
-        price = ticket.total_price(user, hashed_paid)
+        price = ticket.total_price(user, paid: paid)
         if result
           result +=  price unless price.zero?
         else

app/models/ticket_purchase.rb

@@ -13,6 +13,8 @@ class TicketPurchase < ActiveRecord::Base
   delegate :price_cents, to: :ticket
   delegate :price_currency, to: :ticket
 
+  scope :unpaid, -> { where(paid: false) }
+
   def self.purchase(conference, user, purchases)
     errors = []
     ActiveRecord::Base.transaction do