我通过自制软件安装了mysql。创建了数据库prestamoApp_development我启动了服务器（mysql.server start），然后创建了一个新的空项目（rails new prestamoApp -d mysql）。 最后在database.yml文件中配置了用户和密码，如下所示：
default: &default
adapter: mysql2
encoding: utf8
pool: 5
username: root
password: pqdsimple
socket: /tmp/mysql.sock
development:
<<: *default
database: prestamoApp_development
# Warning: The database defined as "test" will be erased and
# re-generated from your development database when you run "rake".
# Do not set this db to the same as development or production.
test:
<<: *default
database: prestamoApp_test
# As with config/secrets.yml, you never want to store sensitive information,
# like your database password, in your source code. If your source code is
# ever seen by anyone, they now have access to your database.
#
# Instead, provide the password as a unix environment variable when you boot
# the app. Read http://guides.rubyonrails.org/configuring.html#configuring-a-database
# for a full rundown on how to provide these environment variables in a
# production deployment.
#
# On Heroku and other platform providers, you may have a full connection URL
# available as an environment variable. For example:
#
# DATABASE_URL="mysql2://myuser:mypass@localhost/somedatabase"
#
# You can use this database configuration with:
#
# production:
# url: <%= ENV['DATABASE_URL'] %>
#
production:
<<: *default
database: prestamoApp_production
username: prestamoApp
password: <%= ENV['PRESTAMOAPP_DATABASE_PASSWORD'] %>
这是宝石文件：
source 'https://rubygems.org'
# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '4.2.4'
# Use mysql as the database for Active Record
gem 'mysql2'
# Use SCSS for stylesheets
gem 'sass-rails', '~> 5.0'
# Use Uglifier as compressor for JavaScript assets
gem 'uglifier', '>= 1.3.0'
# Use CoffeeScript for .coffee assets and views
gem 'coffee-rails', '~> 4.1.0'
# See https://github.com/rails/execjs#readme for more supported runtimes
# gem 'therubyracer', platforms: :ruby
# Use jquery as the JavaScript library
gem 'jquery-rails'
# Turbolinks makes following links in your web application faster. Read more: https://github.com/rails/turbolinks
gem 'turbolinks'
# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
gem 'jbuilder', '~> 2.0'
# bundle exec rake doc:rails generates the API under doc/api.
gem 'sdoc', '~> 0.4.0', group: :doc
# Use ActiveModel has_secure_password
# gem 'bcrypt', '~> 3.1.7'
# Use Unicorn as the app server
# gem 'unicorn'
# 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'
end
group :development do
# Access an IRB console on exception pages or by using <%= console %> in views
gem 'web-console', '~> 2.0'
# Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
gem 'spring'
end
以防万一gem.lock：
GEM
remote: https://rubygems.org/
specs:
actionmailer (4.2.4)
actionpack (= 4.2.4)
actionview (= 4.2.4)
activejob (= 4.2.4)
mail (~> 2.5, >= 2.5.4)
rails-dom-testing (~> 1.0, >= 1.0.5)
actionpack (4.2.4)
actionview (= 4.2.4)
activesupport (= 4.2.4)
rack (~> 1.6)
rack-test (~> 0.6.2)
rails-dom-testing (~> 1.0, >= 1.0.5)
rails-html-sanitizer (~> 1.0, >= 1.0.2)
actionview (4.2.4)
activesupport (= 4.2.4)
builder (~> 3.1)
erubis (~> 2.7.0)
rails-dom-testing (~> 1.0, >= 1.0.5)
rails-html-sanitizer (~> 1.0, >= 1.0.2)
activejob (4.2.4)
activesupport (= 4.2.4)
globalid (>= 0.3.0)
activemodel (4.2.4)
activesupport (= 4.2.4)
builder (~> 3.1)
activerecord (4.2.4)
activemodel (= 4.2.4)
activesupport (= 4.2.4)
arel (~> 6.0)
activesupport (4.2.4)
i18n (~> 0.7)
json (~> 1.7, >= 1.7.7)
minitest (~> 5.1)
thread_safe (~> 0.3, >= 0.3.4)
tzinfo (~> 1.1)
arel (6.0.3)
binding_of_caller (0.7.2)
debug_inspector (>= 0.0.1)
builder (3.2.2)
byebug (6.0.2)
coffee-rails (4.1.0)
coffee-script (>= 2.2.0)
railties (>= 4.0.0, < 5.0)
coffee-script (2.4.1)
coffee-script-source
execjs
coffee-script-source (1.9.1.1)
debug_inspector (0.0.2)
erubis (2.7.0)
execjs (2.6.0)
globalid (0.3.6)
activesupport (>= 4.1.0)
i18n (0.7.0)
jbuilder (2.3.1)
activesupport (>= 3.0.0, < 5)
multi_json (~> 1.2)
jquery-rails (4.0.5)
rails-dom-testing (~> 1.0)
railties (>= 4.2.0)
thor (>= 0.14, < 2.0)
json (1.8.3)
loofah (2.0.3)
nokogiri (>= 1.5.9)
mail (2.6.3)
mime-types (>= 1.16, < 3)
mime-types (2.6.1)
mini_portile (0.6.2)
minitest (5.8.0)
multi_json (1.11.2)
mysql2 (0.4.0)
nokogiri (1.6.6.2)
mini_portile (~> 0.6.0)
rack (1.6.4)
rack-test (0.6.3)
rack (>= 1.0)
rails (4.2.4)
actionmailer (= 4.2.4)
actionpack (= 4.2.4)
actionview (= 4.2.4)
activejob (= 4.2.4)
activemodel (= 4.2.4)
activerecord (= 4.2.4)
activesupport (= 4.2.4)
bundler (>= 1.3.0, < 2.0)
railties (= 4.2.4)
sprockets-rails
rails-deprecated_sanitizer (1.0.3)
activesupport (>= 4.2.0.alpha)
rails-dom-testing (1.0.7)
activesupport (>= 4.2.0.beta, < 5.0)
nokogiri (~> 1.6.0)
rails-deprecated_sanitizer (>= 1.0.1)
rails-html-sanitizer (1.0.2)
loofah (~> 2.0)
railties (4.2.4)
actionpack (= 4.2.4)
activesupport (= 4.2.4)
rake (>= 0.8.7)
thor (>= 0.18.1, < 2.0)
rake (10.4.2)
rdoc (4.2.0)
sass (3.4.18)
sass-rails (5.0.4)
railties (>= 4.0.0, < 5.0)
sass (~> 3.1)
sprockets (>= 2.8, < 4.0)
sprockets-rails (>= 2.0, < 4.0)
tilt (>= 1.1, < 3)
sdoc (0.4.1)
json (~> 1.7, >= 1.7.7)
rdoc (~> 4.0)
spring (1.3.6)
sprockets (3.3.4)
rack (~> 1.0)
sprockets-rails (2.3.3)
actionpack (>= 3.0)
activesupport (>= 3.0)
sprockets (>= 2.8, < 4.0)
thor (0.19.1)
thread_safe (0.3.5)
tilt (2.0.1)
turbolinks (2.5.3)
coffee-rails
tzinfo (1.2.2)
thread_safe (~> 0.1)
uglifier (2.7.2)
execjs (>= 0.3.0)
json (>= 1.8.0)
web-console (2.2.1)
activemodel (>= 4.0)
binding_of_caller (>= 0.7.2)
railties (>= 4.0)
sprockets-rails (>= 2.0, < 4.0)
PLATFORMS
ruby
DEPENDENCIES
byebug
coffee-rails (~> 4.1.0)
jbuilder (~> 2.0)
jquery-rails
mysql2
rails (= 4.2.4)
sass-rails (~> 5.0)
sdoc (~> 0.4.0)
spring
turbolinks
uglifier (>= 1.3.0)
web-console (~> 2.0)
BUNDLED WITH
1.10.6
当我尝试这样做时：
rake db:schema:dump
我收到此错误：
rake aborted!
Gem::LoadError: 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).
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/connection_adapters/connection_specification.rb:177:in `rescue in spec'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/connection_adapters/connection_specification.rb:174:in `spec'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/connection_handling.rb:50:in `establish_connection'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/railtie.rb:120:in `block (2 levels) in <class:Railtie>'
/usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.4/lib/active_support/lazy_load_hooks.rb:38:in `instance_eval'
/usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.4/lib/active_support/lazy_load_hooks.rb:38:in `execute_hook'
/usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.4/lib/active_support/lazy_load_hooks.rb:45:in `block in run_load_hooks'
/usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.4/lib/active_support/lazy_load_hooks.rb:44:in `each'
/usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.4/lib/active_support/lazy_load_hooks.rb:44:in `run_load_hooks'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/base.rb:315:in `<module:ActiveRecord>'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/base.rb:26:in `<top (required)>'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/tasks/mysql_database_tasks.rb:8:in `<class:MySQLDatabaseTasks>'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/tasks/mysql_database_tasks.rb:3:in `<module:Tasks>'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/tasks/mysql_database_tasks.rb:2:in `<module:ActiveRecord>'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/tasks/mysql_database_tasks.rb:1:in `<top (required)>'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/tasks/database_tasks.rb:50:in `<module:DatabaseTasks>'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/tasks/database_tasks.rb:37:in `<module:Tasks>'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/tasks/database_tasks.rb:4:in `<module:ActiveRecord>'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/tasks/database_tasks.rb:3:in `<top (required)>'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/railtie.rb:41:in `block (3 levels) in <class:Railtie>'
Gem::LoadError: can't activate mysql2 (~> 0.3.13), already activated mysql2-0.4.0. Make sure all dependencies are added to Gemfile.
/usr/local/lib/ruby/gems/2.2.0/gems/bundler-1.10.6/lib/bundler/rubygems_integration.rb:302:in `block in replace_gem'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/connection_adapters/mysql2_adapter.rb:3:in `<top (required)>'
/usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.4/lib/active_support/dependencies.rb:274:in `require'
/usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.4/lib/active_support/dependencies.rb:274:in `block in require'
/usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.4/lib/active_support/dependencies.rb:240:in `load_dependency'
/usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.4/lib/active_support/dependencies.rb:274:in `require'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/connection_adapters/connection_specification.rb:175:in `spec'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/connection_handling.rb:50:in `establish_connection'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/railtie.rb:120:in `block (2 levels) in <class:Railtie>'
/usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.4/lib/active_support/lazy_load_hooks.rb:38:in `instance_eval'
/usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.4/lib/active_support/lazy_load_hooks.rb:38:in `execute_hook'
/usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.4/lib/active_support/lazy_load_hooks.rb:45:in `block in run_load_hooks'
/usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.4/lib/active_support/lazy_load_hooks.rb:44:in `each'
/usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.4/lib/active_support/lazy_load_hooks.rb:44:in `run_load_hooks'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/base.rb:315:in `<module:ActiveRecord>'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/base.rb:26:in `<top (required)>'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/tasks/mysql_database_tasks.rb:8:in `<class:MySQLDatabaseTasks>'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/tasks/mysql_database_tasks.rb:3:in `<module:Tasks>'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/tasks/mysql_database_tasks.rb:2:in `<module:ActiveRecord>'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/tasks/mysql_database_tasks.rb:1:in `<top (required)>'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/tasks/database_tasks.rb:50:in `<module:DatabaseTasks>'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/tasks/database_tasks.rb:37:in `<module:Tasks>'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/tasks/database_tasks.rb:4:in `<module:ActiveRecord>'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/tasks/database_tasks.rb:3:in `<top (required)>'
/usr/local/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/railtie.rb:41:in `block (3 levels) in <class:Railtie>'
Tasks: TOP => db:schema:dump => db:load_config
(See full trace by running task with --trace)
有人可以告诉我有什么问题吗？为了解决这个问题，我需要做什么，除了创建一个空项目，我什么都没做。
答案 0 :(得分：41)
刚刚降级mysql gem gem 'mysql2', '~> 0.3.20'
这是bug
答案 1 :(得分：3)
对于新编码员（像我一样）gem 'mysql2', '~> 0.3.20'不是命令！
如果你尝试，你将收到错误“未知命令mysql2”。
vi /编辑Gemfile并追加 替换
gem 'mysql2'
与
gem 'mysql2', '~> 0.3.20'
保存
然后输入命令：
bundle install
然后重试rake db:________命令。