Error: Permission Denied @ rb_sysopen when trying to install gem pg in my rails app (Windows 7)

Question

Using windows 7 ultimate and running cmd as administrator.
I have recently started working with ruby on rails and in my new rails app I'm attempting to install postgres(pg) gem and then I get a permission denied error as shown below:

C:\wamp\www\RailsApp>bundle install
DL is deprecated, please use Fiddle
Fetching gem metadata from https://rubygems.org/...........
Using rake 10.4.2
Using i18n 0.7.0
Using json 1.8.2
Using minitest 5.5.1
Using thread_safe 0.3.4
Using tzinfo 1.2.2
Using activesupport 4.1.8
Using builder 3.2.2
Using erubis 2.7.0
Using actionview 4.1.8
Using rack 1.5.2
Using rack-test 0.6.3
Using actionpack 4.1.8
Using mime-types 2.4.3
Using mail 2.6.3
Using actionmailer 4.1.8
Using activemodel 4.1.8
Using arel 5.0.1.20140414130214
Using activerecord 4.1.8
Using coffee-script-source 1.8.0
Using execjs 2.2.2
Using coffee-script 2.3.0
Using thor 0.19.1
Using railties 4.1.8
Using coffee-rails 4.0.1
Using hike 1.2.3
Using multi_json 1.10.1
Using jbuilder 2.2.6
Using jquery-rails 3.1.2

Errno::EACCES: Permission denied @ rb_sysopen - C:/Ruby21/lib/ruby/gems/2.1.0/ge
ms/pg-0.18.1-x86-mingw32/lib/2.1/pg_ext.so
An error occurred while installing pg (0.18.1), and Bundler cannot continue.
Make sure that `gem install pg -v '0.18.1'` succeeds before bundling.

I have tried

gem install pg

which leads to the same error.
However... when I try

gem install pg -mingw32

It appears successfull:

C:\wamp\www\RailsApp>gem install pg -mingw32
Fetching: pg-0.18.1-x86-mingw32.gem (100%)
Successfully installed pg-0.18.1-x86-mingw32
invalid options: -f fivefish
(invalid options are ignored)
Parsing documentation for pg-0.18.1-x86-mingw32
Installing ri documentation for pg-0.18.1-x86-mingw32
Done installing documentation for pg after 16 seconds
1 gem installed

Though after searching through several posts I fail to find a solution to how I can implement this with the gemfile to make the bundler run without errors nor find a solution to why I get the permission denied error in the first place.
Here is my current gemfile:

source 'https://rubygems.org'


# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '4.1.8'
# Use postgresql as the database for Active Record
gem 'pg'
# Use SCSS for stylesheets
gem 'sass-rails', '~> 4.0.3'
# Use Uglifier as compressor for JavaScript assets
gem 'uglifier', '>= 1.3.0'
# Use CoffeeScript for .js.coffee assets and views
gem 'coffee-rails', '~> 4.0.0'
# See https://github.com/sstephenson/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

# Use debugger
# gem 'debugger', group: [:development, :test]

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

Note: I have tried using

gem 'pg', platform: :mingw

yet, the error still occurs.
Hope this helps as to make my problem clear and feel free to ask and I'll supply more data possible.
Any suggestions are valued and accepted, thank you.

Answer 1

If you are on windows, use thisgem install pg -mingw32

Answer 2

It looks like bundle is trying to install things in the

C:/Ruby21/lib/ruby/gems/2.1.0/ge
ms/pg-0.18.1-x86-mingw32/lib/2.1/

directory and your user (or the process that is executing bundle) does not have permission to write things to that particular directory.

I am no expert on bundle, but one troubleshooting step you could try is to run a windows command prompt as administrator explicitly. You can do this by searching for CMD, then right-clicking on it and selecting "run as administrator". This sometimes corrects file permissions issues when installing things in windows.