diff --git a/Procfile b/Procfile
index 84cab0c9..8b9e764c 100644
--- a/Procfile
+++ b/Procfile
@@ -1,2 +1,2 @@
-web: bundle exec puma -C config/puma.rb
+web: bundle exec rails server -b 0.0.0.0
 worker: bundle exec rails jobs:work
diff --git a/app/controllers/admin/commercials_controller.rb b/app/controllers/admin/commercials_controller.rb
index 003e04bf..0680b436 100644
--- a/app/controllers/admin/commercials_controller.rb
+++ b/app/controllers/admin/commercials_controller.rb
@@ -70,7 +70,7 @@ module Admin
 
         flash[:error] = errors_text
       end
-      redirect_to :back
+      redirect_back(fallback_location: root_path)
     end
 
     private
diff --git a/app/controllers/openids_controller.rb b/app/controllers/openids_controller.rb
index 3bccba2f..82af86d0 100644
--- a/app/controllers/openids_controller.rb
+++ b/app/controllers/openids_controller.rb
@@ -6,6 +6,6 @@ class OpenidsController < ApplicationController
 
   def destroy
     @openid.destroy
-    redirect_to :back
+    redirect_back(fallback_location: root_path)
   end
 end
diff --git a/app/controllers/proposals_controller.rb b/app/controllers/proposals_controller.rb
index d59cc2d8..692b4db0 100644
--- a/app/controllers/proposals_controller.rb
+++ b/app/controllers/proposals_controller.rb
@@ -104,7 +104,7 @@ class ProposalsController < ApplicationController
         @event.event_schedules.destroy_all
       end
     rescue Transitions::InvalidTransition
-      redirect_to :back, error: "Event can't be withdrawn"
+      redirect_back(fallback_location: root_path, error: "Event can't be withdrawn")
       return
     end
 
@@ -124,7 +124,7 @@ class ProposalsController < ApplicationController
     begin
       @event.confirm
     rescue Transitions::InvalidTransition
-      redirect_to :back, error: "Event can't be confirmed"
+      redirect_back(fallback_location: root_path, error: "Event can't be confirmed")
       return
     end
 
diff --git a/app/controllers/surveys_controller.rb b/app/controllers/surveys_controller.rb
index 5c0af57e..349bc34b 100644
--- a/app/controllers/surveys_controller.rb
+++ b/app/controllers/surveys_controller.rb
@@ -40,6 +40,6 @@ class SurveysController < ApplicationController
       end
     end
 
-    redirect_to :back
+    redirect_back(fallback_location: root_path)
   end
 end
diff --git a/app/controllers/users/omniauth_callbacks_controller.rb b/app/controllers/users/omniauth_callbacks_controller.rb
index 03ae4044..8d0f80dd 100644
--- a/app/controllers/users/omniauth_callbacks_controller.rb
+++ b/app/controllers/users/omniauth_callbacks_controller.rb
@@ -2,7 +2,7 @@
 
 module Users
   class OmniauthCallbacksController < Devise::OmniauthCallbacksController
-    skip_before_filter :verify_authenticity_token
+    skip_before_action :verify_authenticity_token
     skip_authorization_check
 
     User.omniauth_providers.each do |provider|
diff --git a/app/helpers/application_helper.rb b/app/helpers/application_helper.rb
index 36adbb0b..c8b9acbe 100644
--- a/app/helpers/application_helper.rb
+++ b/app/helpers/application_helper.rb
@@ -88,10 +88,9 @@ module ApplicationHelper
     end
   end
 
-  # Same as redirect_to(:back) if there is a valid HTTP referer, otherwise redirect_to()
   def redirect_back_or_to(options = {}, response_status = {})
     if request.env['HTTP_REFERER']
-      redirect_to :back
+      redirect_back(fallback_location: root_path)
     else
       redirect_to options, response_status
     end
diff --git a/app/models/conference.rb b/app/models/conference.rb
index 89ca5d19..ca37e6d0 100644
--- a/app/models/conference.rb
+++ b/app/models/conference.rb
@@ -1120,7 +1120,6 @@ class Conference < ApplicationRecord
   #
   def create_email_settings
     build_email_settings
-    true
   end
 
   ##
diff --git a/app/models/program.rb b/app/models/program.rb
index 6c3a3b4a..0dadee15 100644
--- a/app/models/program.rb
+++ b/app/models/program.rb
@@ -211,7 +211,6 @@ class Program < ApplicationRecord
     EventType.create(title: 'Workshop', length: 60, color: '#0000FF', description: 'Interactive hands-on practice',
                      minimum_abstract_length: 0,
                      maximum_abstract_length: 500, program_id: id)
-    true
   end
 
   ##
@@ -227,7 +226,6 @@ class Program < ApplicationRecord
     DifficultyLevel.create(title: 'Hard',
                            description: 'Events require expert knowledge of the topic.',
                            color: '#EF6E69', program_id: id)
-    true
   end
 
   ##
diff --git a/app/models/registration.rb b/app/models/registration.rb
index f7c032ea..a071af28 100644
--- a/app/models/registration.rb
+++ b/app/models/registration.rb
@@ -76,7 +76,7 @@ class Registration < ApplicationRecord
   end
 
   def registration_limit_not_exceed
-    if conference.registration_limit > 0 && conference.registrations(:reload).count >= conference.registration_limit
+    if conference.registration_limit > 0 && conference.registrations.count >= conference.registration_limit
       errors.add(:base, 'Registration limit exceeded')
     end
   end
diff --git a/app/models/role.rb b/app/models/role.rb
index d759f47d..2be87d7b 100644
--- a/app/models/role.rb
+++ b/app/models/role.rb
@@ -18,6 +18,6 @@ class Role < ApplicationRecord
 
   # Needed to ensure that removing all user from role doesn't remove role.
   def cancel
-    false
+    throw(:abort)
   end
 end
diff --git a/app/models/user.rb b/app/models/user.rb
index bc71f61f..ec3132e1 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -11,6 +11,7 @@ class User < ApplicationRecord
   # prevent N+1 queries with has_cached_role? by preloading roles *always*
   default_scope { preload(:roles) }
 
+  has_many :ticket_purchases, dependent: :destroy
   has_many :physical_tickets, through: :ticket_purchases do
     def by_conference(conference)
       where('ticket_purchases.conference_id = ?', conference)
@@ -67,7 +68,6 @@ class User < ApplicationRecord
     end
   end
   has_many :events_registrations, through: :registrations
-  has_many :ticket_purchases, dependent: :destroy
   has_many :payments, dependent: :destroy
   has_many :tickets, through: :ticket_purchases, source: :ticket do
     def for_registration conference
diff --git a/config/initializers/new_framework_defaults.rb b/config/initializers/new_framework_defaults.rb
index cbf423a8..534a2396 100644
--- a/config/initializers/new_framework_defaults.rb
+++ b/config/initializers/new_framework_defaults.rb
@@ -20,6 +20,3 @@ ActiveSupport.to_time_preserves_timezone = false
 
 # Require `belongs_to` associations by default. Previous versions had false.
 Rails.application.config.active_record.belongs_to_required_by_default = false
-
-# Do not halt callback chains when a callback returns false. Previous versions had true.
-ActiveSupport.halt_callback_chains_on_return_false = true
diff --git a/config/puma.rb b/config/puma.rb
index fa11fb8f..9886e1e5 100644
--- a/config/puma.rb
+++ b/config/puma.rb
@@ -21,7 +21,7 @@ environment ENV.fetch("RAILS_ENV") { "development" }
 # Workers do not work on JRuby or Windows (both of which do not support
 # processes).
 #
-workers ENV.fetch("WEB_CONCURRENCY") { 2 }
+workers ENV.fetch("WEB_CONCURRENCY") { 1 }
 
 # Use the `preload_app!` method when specifying a `workers` number.
 # This directive tells Puma to first boot the application and load code
diff --git a/spec/models/role_spec.rb b/spec/models/role_spec.rb
index 1d2c60a4..58c2e287 100644
--- a/spec/models/role_spec.rb
+++ b/spec/models/role_spec.rb
@@ -4,8 +4,8 @@ require 'spec_helper'
 
 describe Role do
   let(:conference) { create(:conference) }
-  let!(:organizer_role) { Role.find_by(name: 'organizer', resource: conference) }
-  let!(:cfp_role) { Role.find_by(name: 'cfp', resource: conference) }
+  let!(:organizer_role) { Role.find_or_create_by(name: 'organizer', resource: conference) }
+  let!(:cfp_role) { Role.find_or_create_by(name: 'cfp', resource: conference) }
   let!(:organizer) { create(:organizer, resource: conference) }
   let(:user) { create(:user) }