Gem::LoadError：无法激活 pg (~> 0.18)，已经激活 pg-1.0.0 [重复]

Question

我一直在学习 Rails 教程，发现 here 并成功地使用 $ rails db:migrate 迁移评论迁移。在此之前，我已经能够生成文章模型并迁移文章创建迁移没有问题。在这两次迁移之间，我的 Gemfile 没有任何变化，所以我不确定 Bundler 有什么问题。

以下是错误，后面是完整的命令行输出，以及我的 Gemfile 和 schema.rb：

Gem::LoadError: can't activate pg (~> 0.18), already activated pg-1.0.0.

Gem::LoadError: Specified 'postgresql' for database adapter, but the gem is not loaded. Add `gem 'pg'` to your Gemfile (and ensure its version is at the minimum required by ActiveRecord).

完整的命令行输出

xxx:gangelo: ~/dev/rails/test/blog (master*) ☠  rbenv exec rails db:migrate
rails aborted!
Gem::LoadError: Specified 'postgresql' for database adapter, but the gem is not loaded. Add `gem 'pg'` to your Gemfile (and ensure its version is at the minimum required by ActiveRecord).
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/connection_adapters/connection_specification.rb:188:in `rescue in spec'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/connection_adapters/connection_specification.rb:185:in `spec'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/connection_adapters/abstract/connection_pool.rb:880:in `establish_connection'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/connection_handling.rb:58:in `establish_connection'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/railtie.rb:124:in `block (2 levels) in <class:Railtie>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/lazy_load_hooks.rb:69:in `instance_eval'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/lazy_load_hooks.rb:69:in `block in execute_hook'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/lazy_load_hooks.rb:60:in `with_execution_control'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/lazy_load_hooks.rb:65:in `execute_hook'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/lazy_load_hooks.rb:50:in `block in run_load_hooks'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/lazy_load_hooks.rb:49:in `each'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/lazy_load_hooks.rb:49:in `run_load_hooks'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/base.rb:326:in `<module:ActiveRecord>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/base.rb:25:in `<top (required)>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `require'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `block in require'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:258:in `load_dependency'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `require'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/tasks/mysql_database_tasks.rb:6:in `<class:MySQLDatabaseTasks>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/tasks/mysql_database_tasks.rb:3:in `<module:Tasks>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/tasks/mysql_database_tasks.rb:2:in `<module:ActiveRecord>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/tasks/mysql_database_tasks.rb:1:in `<top (required)>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `require'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `block in require'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:258:in `load_dependency'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `require'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/tasks/database_tasks.rb:74:in `<module:DatabaseTasks>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/tasks/database_tasks.rb:35:in `<module:Tasks>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/tasks/database_tasks.rb:2:in `<module:ActiveRecord>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/tasks/database_tasks.rb:1:in `<top (required)>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `require'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `block in require'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:258:in `load_dependency'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `require'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/railtie.rb:34:in `block (3 levels) in <class:Railtie>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/railties-5.1.4/lib/rails/commands/rake/rake_command.rb:21:in `block in perform'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/railties-5.1.4/lib/rails/commands/rake/rake_command.rb:18:in `perform'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/railties-5.1.4/lib/rails/command.rb:46:in `invoke'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/railties-5.1.4/lib/rails/commands.rb:16:in `<top (required)>'
/Users/gangelo/dev/rails/test/blog/bin/rails:9:in `require'
/Users/gangelo/dev/rails/test/blog/bin/rails:9:in `<top (required)>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/spring-2.0.2/lib/spring/client/rails.rb:28:in `load'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/spring-2.0.2/lib/spring/client/rails.rb:28:in `call'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/spring-2.0.2/lib/spring/client/command.rb:7:in `call'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/spring-2.0.2/lib/spring/client.rb:30:in `run'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/spring-2.0.2/bin/spring:49:in `<top (required)>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/spring-2.0.2/lib/spring/binstub.rb:31:in `load'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/spring-2.0.2/lib/spring/binstub.rb:31:in `<top (required)>'
/Users/gangelo/dev/rails/test/blog/bin/spring:15:in `<top (required)>'
bin/rails:3:in `load'
bin/rails:3:in `<main>'

Caused by:
Gem::LoadError: can't activate pg (~> 0.18), already activated pg-1.0.0. Make sure all dependencies are added to Gemfile.
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/connection_adapters/postgresql_adapter.rb:2:in `<top (required)>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `require'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `block in require'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:258:in `load_dependency'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `require'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/connection_adapters/connection_specification.rb:186:in `spec'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/connection_adapters/abstract/connection_pool.rb:880:in `establish_connection'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/connection_handling.rb:58:in `establish_connection'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/railtie.rb:124:in `block (2 levels) in <class:Railtie>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/lazy_load_hooks.rb:69:in `instance_eval'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/lazy_load_hooks.rb:69:in `block in execute_hook'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/lazy_load_hooks.rb:60:in `with_execution_control'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/lazy_load_hooks.rb:65:in `execute_hook'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/lazy_load_hooks.rb:50:in `block in run_load_hooks'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/lazy_load_hooks.rb:49:in `each'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/lazy_load_hooks.rb:49:in `run_load_hooks'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/base.rb:326:in `<module:ActiveRecord>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/base.rb:25:in `<top (required)>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `require'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `block in require'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:258:in `load_dependency'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `require'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/tasks/mysql_database_tasks.rb:6:in `<class:MySQLDatabaseTasks>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/tasks/mysql_database_tasks.rb:3:in `<module:Tasks>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/tasks/mysql_database_tasks.rb:2:in `<module:ActiveRecord>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/tasks/mysql_database_tasks.rb:1:in `<top (required)>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `require'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `block in require'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:258:in `load_dependency'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `require'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/tasks/database_tasks.rb:74:in `<module:DatabaseTasks>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/tasks/database_tasks.rb:35:in `<module:Tasks>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/tasks/database_tasks.rb:2:in `<module:ActiveRecord>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/tasks/database_tasks.rb:1:in `<top (required)>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `require'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `block in require'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:258:in `load_dependency'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `require'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/activerecord-5.1.4/lib/active_record/railtie.rb:34:in `block (3 levels) in <class:Railtie>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/railties-5.1.4/lib/rails/commands/rake/rake_command.rb:21:in `block in perform'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/railties-5.1.4/lib/rails/commands/rake/rake_command.rb:18:in `perform'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/railties-5.1.4/lib/rails/command.rb:46:in `invoke'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/railties-5.1.4/lib/rails/commands.rb:16:in `<top (required)>'
/Users/gangelo/dev/rails/test/blog/bin/rails:9:in `require'
/Users/gangelo/dev/rails/test/blog/bin/rails:9:in `<top (required)>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/spring-2.0.2/lib/spring/client/rails.rb:28:in `load'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/spring-2.0.2/lib/spring/client/rails.rb:28:in `call'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/spring-2.0.2/lib/spring/client/command.rb:7:in `call'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/spring-2.0.2/lib/spring/client.rb:30:in `run'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/spring-2.0.2/bin/spring:49:in `<top (required)>'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/spring-2.0.2/lib/spring/binstub.rb:31:in `load'
/Users/gangelo/dev/rails/test/blog/vendor/bundle/gems/spring-2.0.2/lib/spring/binstub.rb:31:in `<top (required)>'
/Users/gangelo/dev/rails/test/blog/bin/spring:15:in `<top (required)>'
bin/rails:3:in `load'
bin/rails:3:in `<main>'
Tasks: TOP => db:migrate => db:load_config
(See full trace by running task with --trace)

宝石文件

source 'https://rubygems.org'

ruby '2.3.1'

git_source(:github) do |repo_name|
  repo_name = "#{repo_name}/#{repo_name}" unless repo_name.include?("/")
  "https://github.com/#{repo_name}.git"
end


# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '~> 5.1.4'

# Use sqlite3 as the database for Active Record
# gem 'sqlite3'

# Use postgres as the database for Active Record
gem 'pg'

# Use Puma as the app server
gem 'puma', '~> 3.7'
# Use SCSS for stylesheets
gem 'sass-rails', '~> 5.0'
# Use Uglifier as compressor for JavaScript assets
gem 'uglifier', '>= 1.3.0'
# See https://github.com/rails/execjs#readme for more supported runtimes
# gem 'therubyracer', platforms: :ruby

# Use CoffeeScript for .coffee assets and views
gem 'coffee-rails', '~> 4.2'
# Turbolinks makes navigating your web application faster. Read more: https://github.com/turbolinks/turbolinks
gem 'turbolinks', '~> 5'
# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
gem 'jbuilder', '~> 2.5'
# Use Redis adapter to run Action Cable in production
# gem 'redis', '~> 3.0'
# Use ActiveModel has_secure_password
# gem 'bcrypt', '~> 3.1.7'

# Use Capistrano for deployment
# gem 'capistrano-rails', group: :development

group :development, :test do
  # Call 'byebug' anywhere in the code to stop execution and get a debugger console
  gem 'byebug', platforms: [:mri, :mingw, :x64_mingw]
  # Adds support for Capybara system testing and selenium driver
  gem 'capybara', '~> 2.13'
  gem 'selenium-webdriver'
end

# gma - start
group :development, :test do
  gem 'rspec-rails', '~> 3.5', '>= 3.5.2'
  gem 'rspec-activemodel-mocks', '~> 1.0', '>= 1.0.3'
  gem 'shoulda-matchers', '~> 3.1', '>= 3.1.1'
  gem 'factory_bot_rails', '~> 4.8', '>= 4.8.2'
  gem 'ffaker', '~> 2.2'
  # gem 'timecop', '~> 0.8.1'
end
# gma - end

group :development do
  # Access an IRB console on exception pages or by using <%= console %> anywhere in the code.
  gem 'web-console', '>= 3.3.0'
  gem 'listen', '>= 3.0.5', '< 3.2'
  # Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
  gem 'spring'
  gem 'spring-watcher-listen', '~> 2.0.0'
end

# Windows does not include zoneinfo files, so bundle the tzinfo-data gem
gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]

Schema.rb

ActiveRecord::Schema.define(version: 20180110153949) do
  # These are extensions that must be enabled in order to support this database
  enable_extension "plpgsql"

  create_table "articles", force: :cascade do |t|
    t.string "title"
    t.text "text"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
  end
end

迁移文件

class CreateComments < ActiveRecord::Migration[5.1]
  def change
    create_table :comments do |t|
      t.string :commenter
      t.text :body
      t.references :article, foreign_key: true

      t.timestamps
    end
  end
end

Answer 1

原来 postgres gem 1.0.0 版不与 Rails 5.1.4 兼容。当我将 postgres gem 版本更改为 '~> 0.18' 时，一切正常：

更改自：

gem 'pg' # Defaults to latest version 1.0.0 (not compatible with Rails 5.1.4).

改为：

gem 'pg', '~> 0.18'

Answer 2

目前看来 pg 1.0 与 rails 5.1.4 不兼容。

gem 'pg', '~> 0.18'

0.18 对我有用。