bundle install --without production remediates error, but why? ( Make sure that `gem install pg -v '0.18.1'` succeeds before bundling)

Question

After creating a Ruby on Rails skeleton (before pushing to master and Heroku) and running: bundle install , I sometimes encounter the following error:

An error occurred while installing pg (0.18.2), and Bundler cannot continue Make sure that gem install pg -v '0.18.2' succeeds before bundling.

The following command remediates the issue altogether: bundle install --without production .

Why exactly does the aforementioned command remediate the issue? As I understand, the command bypasses production environment gems for deployment; so, is my understanding correct and why must this be the case? Thank you!

Here is my gemfile

source 'https://rubygems.org'


# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '4.2.0'
# Use sqlite3 as the database for Active Record
# gem 'sqlite3'
# 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/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

group :production do
  gem 'pg'
  gem 'rails_12factor'
end

group :development do
  gem 'sqlite3'
end

gem 'bootstrap-sass'

group :development, :test do
  # Call 'byebug' anywhere in the code to stop execution and get a debugger console
   gem 'byebug'

  # 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

Answer 1

The best resource answering the "why" component of this question appears to be explained thusly by Michael Hartl's Ruby on Rails Tutorial :

"Heroku uses the PostgreSQL database...which means that we need to add the pg gem in the production environment to allow Rails to talk to Postgres...Generally speaking, it's a good idea for the development and production environments to match each other as closely as possible, which includes using the same database, however we'll use SQLite locally and PostgreSQL in production."

So, basically it appears that the issue is a matter of maintaining conventions between two different environments- production and development (local)- and more specifically, database types ( Postgres vs SQLite ), which is why bundle install --without production is required:

"To prepare the system for deployment to production, we run bundle install with a special flag to prevent the local installation of any production gems (which in this case consist of ph and rails_12factor)...Because the only gems added are restricted to a production environment, right now this command doesn't actually install any additional local gems, but it's needed to update Gemfile.lock with the pg and rails_12factor gems."

If the remediation for Heroku deployment bundle install --without production is an acceptable alternative to bundle install , it just seems too bad to be true; if so, is there another setting or file I can revise in order to achieve the same results effected by the regular bundle install ? Thanks!