Answer by okeicalm

app/controllers/tickets_controller.rb

@@ -23,12 +23,20 @@ def show
   end
 
   def edit
+    load_ticket
+    redirect_to root_path, notice: '降車済みの切符です。' if @ticket.used?
   end
 
   def update
-    if @ticket.update(ticket_update_params)
-      redirect_to root_path, notice: '降車しました。😄'
+    load_ticket
+    exited_gate = Gate.find(ticket_update_params[:exited_gate_id])
+
+    if @ticket.used?
+      redirect_to root_path, notice: '降車済みの切符です。' if @ticket.used?
+    elsif exited_gate.exit?(@ticket)
+      update_ticket
     else
+      @ticket.errors[:base] << '降車駅 では降車できません。'
       render :edit
     end
   end
@@ -46,4 +54,12 @@ def ticket_update_params
   def load_ticket
     @ticket = Ticket.find(params[:id])
   end
+
+  def update_ticket
+    if @ticket.update(ticket_update_params)
+      redirect_to root_path, notice: '降車しました。😄'
+    else
+      render :edit
+    end
+  end
 end

app/models/gate.rb

@@ -8,6 +8,27 @@ class Gate < ApplicationRecord
   scope :order_by_station_number, -> { order(:station_number) }
 
   def exit?(ticket)
-    true
+    gate_interval = fetch_gate_interval(ticket.entered_gate_id)
+
+    # 運賃不足ではないかチェック
+    if gate_interval.zero?
+      false
+    elsif FARES[gate_interval - 1] <= ticket.fare
+      true
+    else
+      false
+    end
+  end
+
+  private
+
+  def fetch_gate_interval(entered_gate_id)
+    (
+      entered_station_number(entered_gate_id) - station_number
+    ).abs
+  end
+
+  def entered_station_number(gate_id)
+    Gate.find(gate_id).station_number
   end
 end

app/models/ticket.rb

@@ -3,4 +3,12 @@ class Ticket < ApplicationRecord
   belongs_to :exited_gate, class_name: 'Gate', foreign_key: 'exited_gate_id', required: false
   validates :fare, presence: true, inclusion: Gate::FARES
   validates :entered_gate_id, presence: true
+
+  def used?
+    if exited_gate_id.nil?
+      false
+    else
+      true
+    end
+  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)