heroku run rake > no such file to load -- faker - ruby-on-rails

Question

When running 'heroku run rake' I get this error:
no such file to load -- faker
/app/lib/tasks/sample_data.rake:1:in `require'
/app/lib/tasks/sample_data.rake:1:in `<top (required)>'
I have gem 'faker', '0.3.1' under group :development, :test do in Gemfile.
I have require 'faker' in sample_data.rake
source 'https://rubygems.org'
gem 'rails', '3.2.11'
gem 'gravatar_image_tag', '0.1.0'
group :development, :test do
gem 'sqlite3', '1.3.5'
gem 'rspec-rails', '2.8'
gem 'guard-spork', '1.2.0'
gem 'faker', '0.3.1'
gem 'spork', '0.8.4'
gem 'will_paginate', '3.0'
gem 'webrat', '0.7.1'
gem 'capybara', '1.1.2'
gem 'annotate', '2.5.0'
gem 'factory_girl_rails', '1.0'
end
# Gems used only for assets and not required
# in production environments by default.
group :assets do
gem 'sass-rails', '3.2.5'
gem 'coffee-rails', '3.2.2'
gem 'uglifier', '1.2.3'
end
gem 'jquery-rails', '2.0.2'
group :production do
gem 'pg', '0.12.2'
end

Answer 1

Heroku does not install test or development gems by default.
If you want to load fake in your production, you should remove gem 'faker', '0.3.1' from the group :development, :test do and place it outside any group:
source 'https://rubygems.org'
gem 'rails', '3.2.11'
gem 'gravatar_image_tag', '0.1.0'
gem 'faker', '0.3.1'
group :development, :test do
gem 'sqlite3', '1.3.5'
...
However, if you don't want to load fake, you must ensure that your require fake will be required only when the task is invoked:
task :sample_data => :environment do
require 'faker' #must be inside the task.
...
end
Hope it helps.
EDIT
You can tell Bundler to not load the Gem by:
gem 'faker', '0.3.1', :require => false

Answer 2

I got this issue with the "Rub on Rails 3 Tutorial".
Per gabrielhilals answer, the fix was to move require 'faker' to inside the task
before fix:
require 'faker'
namespace :db do
desc "Fill database with sample data"
task :populate => :environment do
...
...
end
end
after fix:
namespace :db do
require 'faker'
desc "Fill database with sample data"
task :populate => :environment do
...
...
end
end