为什么在运行“捆绑安装”时会出现无法构建 gem 原生扩展错误 pg (0.8.0)？

Question

pg gem is not specified in the gem file, why won't it install and why is it even installing at all if it is not in the gem file. I'm trying to use mysql.

The strange thing is that the pg gem is not in my gemfile, so i thought it might be a dependency of another, after researching dependencies on the gem site, pg is shown not to be a dependency of other gems used.

Error Message:

Installing pg (0.8.0) with native extensions c:/Ruby192/lib/ruby/site_ruby/1.9.1/rubygems/installer.rb:543:in `rescue in block in build_e
xtensions': ERROR: Failed to build gem native extension. (Gem::Installer::ExtensionBuildError)

        c:/Ruby192/bin/ruby.exe extconf.rb
ERROR: can't find pg_config.
HINT: Make sure pg_config is in your PATH
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers.  Check the mkmf.log file for more
details.  You may need configuration options.

Provided configuration options:
        --with-opt-dir
        --without-opt-dir
        --with-opt-include
        --without-opt-include=${opt-dir}/include
        --with-opt-lib
        --without-opt-lib=${opt-dir}/lib
        --with-make-prog

Gem files will remain installed in c:/Ruby192/lib/ruby/gems/1.9.1/gems/pg-0.8.0 for inspection.
Results logged to c:/Ruby192/lib/ruby/gems/1.9.1/gems/pg-0.8.0/ext/gem_make.out
        from c:/Ruby192/lib/ruby/site_ruby/1.9.1/rubygems/installer.rb:521:in `block in build_extensions'
        from c:/Ruby192/lib/ruby/site_ruby/1.9.1/rubygems/installer.rb:496:in `each'
        from c:/Ruby192/lib/ruby/site_ruby/1.9.1/rubygems/installer.rb:496:in `build_extensions'
        from c:/Ruby192/lib/ruby/site_ruby/1.9.1/rubygems/installer.rb:177:in `install'
        from c:/Ruby192/lib/ruby/gems/1.9.1/gems/bundler-1.0.12/lib/bundler/source.rb:96:in `install'
        from c:/Ruby192/lib/ruby/gems/1.9.1/gems/bundler-1.0.12/lib/bundler/installer.rb:55:in `block in run'
        from c:/Ruby192/lib/ruby/gems/1.9.1/gems/bundler-1.0.12/lib/bundler/spec_set.rb:12:in `block in each'
        from c:/Ruby192/lib/ruby/gems/1.9.1/gems/bundler-1.0.12/lib/bundler/spec_set.rb:12:in `each'
        from c:/Ruby192/lib/ruby/gems/1.9.1/gems/bundler-1.0.12/lib/bundler/spec_set.rb:12:in `each'
        from c:/Ruby192/lib/ruby/gems/1.9.1/gems/bundler-1.0.12/lib/bundler/installer.rb:44:in `run'
        from c:/Ruby192/lib/ruby/gems/1.9.1/gems/bundler-1.0.12/lib/bundler/installer.rb:8:in `install'
        from c:/Ruby192/lib/ruby/gems/1.9.1/gems/bundler-1.0.12/lib/bundler/cli.rb:225:in `install'
        from c:/Ruby192/lib/ruby/gems/1.9.1/gems/bundler-1.0.12/lib/bundler/vendor/thor/task.rb:22:in `run'
        from c:/Ruby192/lib/ruby/gems/1.9.1/gems/bundler-1.0.12/lib/bundler/vendor/thor/invocation.rb:118:in `invoke_task'
        from c:/Ruby192/lib/ruby/gems/1.9.1/gems/bundler-1.0.12/lib/bundler/vendor/thor.rb:246:in `dispatch'
        from c:/Ruby192/lib/ruby/gems/1.9.1/gems/bundler-1.0.12/lib/bundler/vendor/thor/base.rb:389:in `start'
        from c:/Ruby192/lib/ruby/gems/1.9.1/gems/bundler-1.0.12/bin/bundle:13:in `<top (required)>'
        from c:/Ruby192/bin/bundle:19:in `load'
        from c:/Ruby192/bin/bundle:19:in `<main>'

and this is the gemfile:

source :rubygems

gem "rails", ">= 3.0"
gem "rack"
gem "clearance", "0.9.0.rc9"
gem "haml"
gem "high_voltage"
gem "hoptoad_notifier"
gem "will_paginate"
gem "formtastic"
gem "dynamic_form"
gem "mysql"

Answer 1

The pg gem is used to access PostgreSQL databases from ruby. Check your Gemfile.lock to find out which gem is requiring it as a dependency.

Please seethis Stack Overflow question if you actually need to use the pg gem.