Rails 5 和 Mysql2 gem 问题

Question

我正在尝试将

Rails 4

应用程序升级到

Rails 5

，看起来一切顺利并且

bundle install

也成功了。然而，当我尝试启动服务器时，我看到以下序列中的两个错误

1. Specified 'mysql2' for database adapter, but the gem is not loaded. Add `gem 'mysql2'` to your Gemfile (and ensure its version is at the minimum required by ActiveRecord). (Gem::LoadError)
2. can't activate mysql2 (>= 0.3.18, < 0.5), already activated mysql2-0.5.5. Make sure all dependencies are added to Gemfile. (Gem::LoadError)

版本详情是

1. Ruby: 2.6.10
2. Rails 5.1
3. bundler 1.17.3

下面是异常日志

/Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activerecord-5.1.0/lib/active_record/connection_adapters/connection_specification.rb:188:in `rescue in spec': Specified 'mysql2' for database adapter, but the gem is not loaded. Add `gem 'mysql2'` to your Gemfile (and ensure its version is at the minimum required by ActiveRecord). (Gem::LoadError)
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activerecord-5.1.0/lib/active_record/connection_adapters/connection_specification.rb:185:in `spec'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activerecord-5.1.0/lib/active_record/connection_adapters/abstract/connection_pool.rb:880:in `establish_connection'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activerecord-5.1.0/lib/active_record/connection_handling.rb:58:in `establish_connection'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activerecord-5.1.0/lib/active_record/railtie.rb:124:in `block (2 levels) in <class:Railtie>'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activesupport-5.1.0/lib/active_support/lazy_load_hooks.rb:45:in `instance_eval'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activesupport-5.1.0/lib/active_support/lazy_load_hooks.rb:45:in `execute_hook'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activesupport-5.1.0/lib/active_support/lazy_load_hooks.rb:35:in `block in on_load'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activesupport-5.1.0/lib/active_support/lazy_load_hooks.rb:34:in `each'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activesupport-5.1.0/lib/active_support/lazy_load_hooks.rb:34:in `on_load'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activerecord-5.1.0/lib/active_record/railtie.rb:120:in `block in <class:Railtie>'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/initializable.rb:30:in `instance_exec'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/initializable.rb:30:in `run'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/initializable.rb:59:in `block in run_initializers'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/2.6.0/tsort.rb:228:in `block in tsort_each'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/2.6.0/tsort.rb:350:in `block (2 levels) in each_strongly_connected_component'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/2.6.0/tsort.rb:431:in `each_strongly_connected_component_from'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/2.6.0/tsort.rb:349:in `block in each_strongly_connected_component'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/2.6.0/tsort.rb:347:in `each'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/2.6.0/tsort.rb:347:in `call'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/2.6.0/tsort.rb:347:in `each_strongly_connected_component'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/2.6.0/tsort.rb:226:in `tsort_each'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/2.6.0/tsort.rb:205:in `tsort_each'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/initializable.rb:58:in `run_initializers'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/application.rb:353:in `initialize!'
    from /Users/dev_user/Desktop/Development/Rails/syzygy/config/environment.rb:5:in `<top (required)>'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activesupport-5.1.0/lib/active_support/dependencies.rb:292:in `require'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activesupport-5.1.0/lib/active_support/dependencies.rb:292:in `block in require'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activesupport-5.1.0/lib/active_support/dependencies.rb:258:in `load_dependency'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activesupport-5.1.0/lib/active_support/dependencies.rb:292:in `require'
    from config.ru:3:in `block in <main>'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rack-2.2.7/lib/rack/builder.rb:116:in `eval'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rack-2.2.7/lib/rack/builder.rb:116:in `new_from_string'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rack-2.2.7/lib/rack/builder.rb:105:in `load_file'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rack-2.2.7/lib/rack/builder.rb:66:in `parse_file'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rack-2.2.7/lib/rack/server.rb:349:in `build_app_and_options_from_config'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rack-2.2.7/lib/rack/server.rb:249:in `app'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/commands/server/server_command.rb:24:in `app'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rack-2.2.7/lib/rack/server.rb:422:in `wrapped_app'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/commands/server/server_command.rb:80:in `log_to_stdout'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/commands/server/server_command.rb:42:in `start'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/commands/server/server_command.rb:130:in `block in perform'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/commands/server/server_command.rb:125:in `tap'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/commands/server/server_command.rb:125:in `perform'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/thor-1.2.1/lib/thor/command.rb:27:in `run'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/thor-1.2.1/lib/thor/invocation.rb:127:in `invoke_command'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/thor-1.2.1/lib/thor.rb:392:in `dispatch'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/command/base.rb:63:in `perform'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/command.rb:44:in `invoke'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/commands.rb:16:in `<top (required)>'
    from bin/rails:4:in `require'
    from bin/rails:4:in `<main>'
/Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/bundler-1.17.3/lib/bundler/rubygems_integration.rb:408:in `block (2 levels) in replace_gem': can't activate mysql2 (>= 0.3.18, < 0.5), already activated mysql2-0.5.5. Make sure all dependencies are added to Gemfile. (Gem::LoadError)
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activerecord-5.1.0/lib/active_record/connection_adapters/mysql2_adapter.rb:4:in `<top (required)>'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activesupport-5.1.0/lib/active_support/dependencies.rb:292:in `require'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activesupport-5.1.0/lib/active_support/dependencies.rb:292:in `block in require'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activesupport-5.1.0/lib/active_support/dependencies.rb:258:in `load_dependency'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activesupport-5.1.0/lib/active_support/dependencies.rb:292:in `require'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activerecord-5.1.0/lib/active_record/connection_adapters/connection_specification.rb:186:in `spec'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activerecord-5.1.0/lib/active_record/connection_adapters/abstract/connection_pool.rb:880:in `establish_connection'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activerecord-5.1.0/lib/active_record/connection_handling.rb:58:in `establish_connection'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activerecord-5.1.0/lib/active_record/railtie.rb:124:in `block (2 levels) in <class:Railtie>'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activesupport-5.1.0/lib/active_support/lazy_load_hooks.rb:45:in `instance_eval'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activesupport-5.1.0/lib/active_support/lazy_load_hooks.rb:45:in `execute_hook'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activesupport-5.1.0/lib/active_support/lazy_load_hooks.rb:35:in `block in on_load'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activesupport-5.1.0/lib/active_support/lazy_load_hooks.rb:34:in `each'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activesupport-5.1.0/lib/active_support/lazy_load_hooks.rb:34:in `on_load'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activerecord-5.1.0/lib/active_record/railtie.rb:120:in `block in <class:Railtie>'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/initializable.rb:30:in `instance_exec'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/initializable.rb:30:in `run'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/initializable.rb:59:in `block in run_initializers'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/2.6.0/tsort.rb:228:in `block in tsort_each'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/2.6.0/tsort.rb:350:in `block (2 levels) in each_strongly_connected_component'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/2.6.0/tsort.rb:431:in `each_strongly_connected_component_from'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/2.6.0/tsort.rb:349:in `block in each_strongly_connected_component'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/2.6.0/tsort.rb:347:in `each'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/2.6.0/tsort.rb:347:in `call'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/2.6.0/tsort.rb:347:in `each_strongly_connected_component'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/2.6.0/tsort.rb:226:in `tsort_each'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/2.6.0/tsort.rb:205:in `tsort_each'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/initializable.rb:58:in `run_initializers'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/application.rb:353:in `initialize!'
    from /Users/dev_user/Desktop/Development/Rails/syzygy/config/environment.rb:5:in `<top (required)>'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activesupport-5.1.0/lib/active_support/dependencies.rb:292:in `require'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activesupport-5.1.0/lib/active_support/dependencies.rb:292:in `block in require'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activesupport-5.1.0/lib/active_support/dependencies.rb:258:in `load_dependency'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/activesupport-5.1.0/lib/active_support/dependencies.rb:292:in `require'
    from config.ru:3:in `block in <main>'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rack-2.2.7/lib/rack/builder.rb:116:in `eval'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rack-2.2.7/lib/rack/builder.rb:116:in `new_from_string'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rack-2.2.7/lib/rack/builder.rb:105:in `load_file'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rack-2.2.7/lib/rack/builder.rb:66:in `parse_file'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rack-2.2.7/lib/rack/server.rb:349:in `build_app_and_options_from_config'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rack-2.2.7/lib/rack/server.rb:249:in `app'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/commands/server/server_command.rb:24:in `app'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rack-2.2.7/lib/rack/server.rb:422:in `wrapped_app'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/commands/server/server_command.rb:80:in `log_to_stdout'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/commands/server/server_command.rb:42:in `start'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/commands/server/server_command.rb:130:in `block in perform'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/commands/server/server_command.rb:125:in `tap'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/commands/server/server_command.rb:125:in `perform'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/thor-1.2.1/lib/thor/command.rb:27:in `run'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/thor-1.2.1/lib/thor/invocation.rb:127:in `invoke_command'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/thor-1.2.1/lib/thor.rb:392:in `dispatch'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/command/base.rb:63:in `perform'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/command.rb:44:in `invoke'
    from /Users/dev_user/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/railties-5.1.0/lib/rails/commands.rb:16:in `<top (required)>'
    from bin/rails:4:in `require'
    from bin/rails:4:in `<main>'

任何关于如何解决这个问题的帮助都会很棒，谢谢