Answer by totto357

app/controllers/tickets_controller.rb

@@ -1,5 +1,6 @@
 class TicketsController < ApplicationController
   before_action :load_ticket, only: %i(edit update show)
+  before_action :used_ticket, only: %i(edit update show)
 
   def index
     redirect_to root_path
@@ -26,9 +27,11 @@ def edit
   end
 
   def update
-    if @ticket.update(ticket_update_params)
+    exited_gate = Gate.find(ticket_update_params[:exited_gate_id])
+    if exited_gate.exit?(@ticket) && @ticket.update(ticket_update_params)
       redirect_to root_path, notice: '降車しました。😄'
     else
+      flash.now[:error] = '降車駅 では降車できません。'
       render :edit
     end
   end
@@ -46,4 +49,8 @@ def ticket_update_params
   def load_ticket
     @ticket = Ticket.find(params[:id])
   end
+
+  def used_ticket
+    redirect_to root_path, notice: '降車済みの切符です。' if @ticket.used?
+  end
 end

app/models/gate.rb

@@ -1,13 +1,43 @@
 # NOTE: Gate = 改札機のイメージ
 class Gate < ApplicationRecord
+
   FARES = [150, 190].freeze
 
   validates :name, presence: true, uniqueness: true
   validates :station_number, presence: true, uniqueness: true
 
   scope :order_by_station_number, -> { order(:station_number) }
 
+  # 指定されたチケットで改札から出られるか
+  # @params [Ticket] ticket 購入したチケット
+  # @return [Boolean] true:出られる
   def exit?(ticket)
-    true
+    return false if self.same_station? ticket&.entered_gate
+    calc_fare(ticket) <= ticket.fare
+  end
+
+  # 同一駅の改札かどうか
+  # @params [Gate] other_gate 比較対象の改札
+  # @return [Boolean] 駅番号が一致する場合にtrue
+  def same_station?(other_gate)
+    return false unless other_gate.instance_of? Gate
+    self.station_number == other_gate.station_number
   end
+
+  private
+
+  # チケットから運賃を計算する
+  # @params [Ticket] ticket 乗車チケット
+  # @return [Integer] 運賃
+  def calc_fare(ticket)
+    FARES[distance_from(ticket.entered_gate) - 1]
+  end
+
+  # 指定された改札がある駅との区間距離を取得する
+  # @params [Gate] gate 改札機
+  # @return [Integer] 改札との区間
+  def distance_from(gate)
+    (station_number - gate.station_number).abs
+  end
+
 end

app/models/ticket.rb

@@ -3,4 +3,10 @@ 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
+
+  # チケットが使用済みかどうか
+  # @return [Boolean] 使用済みの場合true
+  def used?
+    exited_gate.present?
+  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