Do not allow MembershipAppsController#new if already joined

app/controllers/membership_apps_controller.rb

@@ -2,6 +2,7 @@
 
 class MembershipAppsController < ApplicationController
   before_action :set_membership_app, only: :show
+  before_action :verify_not_joined, only: :new
 
   # GET /membership_apps/:id
   def show
@@ -40,4 +41,11 @@ private
   def set_membership_app
     @membership_app = MembershipApp.find params[:id]
   end
+
+  def verify_not_joined
+    return if current_account&.own_membership_app.nil?
+
+    skip_authorization
+    redirect_to current_account.own_membership_app
+  end
 end

spec/requests/membership_apps/new_spec.rb

@@ -3,8 +3,17 @@
 require 'rails_helper'
 
 RSpec.describe 'GET /join' do
+  let!(:membership_app) { create :membership_app, account: owner }
+
+  let(:owner) { create :guest_account }
+
   before do
     sign_in current_account.user if current_account&.user
+
+    if current_account&.guest?
+      get root_url guest_token: current_account.guest_token
+    end
+
     get '/join'
   end
 
@@ -13,4 +22,22 @@ RSpec.describe 'GET /join' do
       expect(response).to have_http_status :ok
     end
   end
+
+  context 'when guest owner is authenticated' do
+    let(:owner) { create :guest_account }
+    let(:current_account) { owner }
+
+    specify do
+      expect(response).to redirect_to membership_app
+    end
+  end
+
+  context 'when usual owner is authenticated' do
+    let(:owner) { create :usual_account }
+    let(:current_account) { owner }
+
+    specify do
+      expect(response).to redirect_to membership_app
+    end
+  end
 end