rails sでcannot load such file -- bundler (LoadError)と言われる。

Question

少し古めのRailsアプリ

Rails 5.2.0と少し古めのRailsアプリを起動させたいです。
しかし、以下のrails serverコマンドを打っても、以下のようなエラーがでます。
RailsやGemの管理の仕組みがわかってなくて、解決方法がわかりません。
どうかお助けしていただけないでしょうか。

rails sすると、cannot load such file -- bundler (LoadError)

$ rails s
Traceback (most recent call last):
        4: from bin/rails:3:in `<main>'
        3: from bin/rails:3:in `load'
        2: from ~/rails/quiz/bin/spring:8:in `<top (required)>'
        1: from ~/.rbenv/versions/2.5.1/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in `require'
~/.rbenv/versions/2.5.1/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in `require': cannot load such file -- bundler (LoadError)

bundle installで、(Gem::GemNotFoundException)

$ bundle instal
Traceback (most recent call last):
        2: from ~/.rbenv/versions/2.5.1/bin/bundle:23:in `<main>'
        1: from ~/.rbenv/versions/2.5.1/lib/ruby/2.5.0/rubygems.rb:308:in `activate_bin_path'
~/.rbenv/versions/2.5.1/lib/ruby/2.5.0/rubygems.rb:289:in `find_spec_for_exe': can't find gem bundler (>= 0.a) with executable bundle (Gem::GemNotFoundException)

バージョン、環境

Gemfile

source 'https://rubygems.org'
git_source(:github) { |repo| "https://github.com/#{repo}.git" }

ruby '2.5.1'

gem 'rails', '~> 5.2.0'

PCはMacで、rbenvを用いて管理してます。

$ rbenv versions
  system
  1.8.6
* 2.5.1 (set by ~/rails/quiz/.ruby-version)
  2.5.8
  2.6.6
  2.7.1
  3.0.0

$ gem list bundler

*** LOCAL GEMS ***

bundler (2.2.14)

Answer 1

[エラーメッセージ] can't find gem bundler (>= 0.a) with executable bundler (Gem::GemNotFoundException) - Qiita
こちらの方法で行けそうでした。

$ gem install bundler -v 1.17.1
Fetching: bundler-1.17.1.gem (100%)
Successfully installed bundler-1.17.1
Parsing documentation for bundler-1.17.1
Installing ri documentation for bundler-1.17.1
Done installing documentation for bundler after 3 seconds
1 gem installed