Ex1〜6 の解答を作成しました。

app/controllers/tickets_controller.rb

@@ -45,5 +45,6 @@ def ticket_update_params
 
   def load_ticket
     @ticket = Ticket.find(params[:id])
+    redirect_to root_path, alert: '降車済みの切符です。' if @ticket.exited_gate_id
   end
 end

app/models/gate.rb

@@ -8,6 +8,14 @@ class Gate < ApplicationRecord
   scope :order_by_station_number, -> { order(:station_number) }
 
   def exit?(ticket)
-    true
+    price = calculate(ticket)
+    price > 0 && ticket.fare - price >= 0
+  end
+
+  private
+
+  def calculate(ticket)
+    section = (station_number - ticket.entered_gate.station_number).abs
+    section > 0 ? FARES[section - 1] : 0
   end
 end

app/models/ticket.rb

@@ -1,6 +1,14 @@
 class Ticket < ApplicationRecord
   belongs_to :entered_gate, class_name: 'Gate', foreign_key: 'entered_gate_id'
-  belongs_to :exited_gate, class_name: 'Gate', foreign_key: 'exited_gate_id', required: false
+  belongs_to :exited_gate, class_name: 'Gate', foreign_key: 'exited_gate_id', optional: true
   validates :fare, presence: true, inclusion: Gate::FARES
   validates :entered_gate_id, presence: true
+
+  validate :must_be_exit, if: :exited_gate_id
+
+  private
+
+  def must_be_exit
+    errors.add(:exited_gate, 'では降車できません。') unless exited_gate.exit?(self)
+  end
 end

test/models/gate_test.rb

@@ -19,7 +19,6 @@ class GateTest < ActiveSupport::TestCase
   end
 
   test 'うめだで150円の切符を買って、みくにで降りる（運賃不足）' do
-    skip 'Please implement this!'
     ticket = Ticket.create!(fare: 150, entered_gate: @umeda)
     refute @mikuni.exit?(ticket)
   end
@@ -46,7 +45,6 @@ class GateTest < ActiveSupport::TestCase
   end
 
   test 'みくにで150円の切符を買って、うめだで降りる（運賃不足）' do
-    skip 'Please implement this!'
     ticket = Ticket.create!(fare: 150, entered_gate: @mikuni)
     refute @umeda.exit?(ticket)
   end
@@ -63,7 +61,6 @@ class GateTest < ActiveSupport::TestCase
 
   # その他
   test '同じ駅では降りられない' do
-    skip 'Please implement this!'
     ticket = Ticket.create!(fare: 190, entered_gate: @umeda)
     refute @umeda.exit?(ticket)
 

test/system/tickets_test.rb

@@ -14,7 +14,6 @@ class TicketsTest < ApplicationSystemTestCase
   end
 
   test '運賃が足りない場合' do
-    skip 'Please implement this!'
     visit root_path
     select '150円', from: '切符'
     select 'うめだ', from: '乗車駅'
@@ -27,7 +26,6 @@ class TicketsTest < ApplicationSystemTestCase
   end
 
   test '同じ駅で降りる場合' do
-    skip 'Please implement this!'
     visit root_path
     select '150円', from: '切符'
     select 'うめだ', from: '乗車駅'
@@ -40,7 +38,6 @@ class TicketsTest < ApplicationSystemTestCase
   end
 
   test 'すでに使用済みの切符を指定されたらトップページに移動する' do
-    skip 'Please implement this!'
     # edit
     ticket = Ticket.create!(fare: 150, entered_gate: gates(:umeda), exited_gate: gates(:juso))
     visit edit_ticket_path(ticket)
@@ -67,4 +64,4 @@ class TicketsTest < ApplicationSystemTestCase
     visit ticket_path(ticket)
     assert_current_path edit_ticket_path(ticket)
   end
-end
+end