Ruby on Rails upgrade Mongoid to version 5.0.0 - ruby-on-rails

Question

I have a Rails application which was using Mongoid 4.0.2. I upgraded Mongoid to version 5.0.0 and start getting this error
....gems/2.2.0/gems/less-rails-2.7.0/lib/less/rails/railtie.rb:19:in `block in <class:Railtie>': undefined method `register_preprocessor' for nil:NilClass (NoMethodError)
How to fix this issue or is any manual how to upgrade Mongoid to version 5.0.0 ?
Gemfile
source 'https://rubygems.org'
# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '4.2.0.beta4'
# Use SCSS for stylesheets
gem 'sass-rails', '~> 5.0.0.beta1'
# 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', '~> 4.0.0.beta2'
# 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
#new gems
#database
gem 'mongoid', '~> 5.0.0.beta'
gem "mongoid-paperclip", :require => "mongoid_paperclip"
gem 'bson_ext'
#stile
gem "therubyracer"
gem "less-rails" #Sprockets (what Rails 3.1 uses for its asset pipeline) supports LESS
gem "twitter-bootstrap-rails"
gem 'bootstrap_form'
gem 'bootstrap-sass', '~> 3.3.4'
gem 'bootswatch-rails'
gem 'simple_form'
gem 'mongo'
gem "figaro"
gem 'sidekiq'
gem 'sidekiq-status'
gem 'rubyzip'
#gem 'sidekiq_status'
#gem 'sidekiq-status'
gem 'sinatra', require: false
gem 'slim'
gem 'capistrano', '~> 3.1.0'
# rails specific capistrano funcitons
gem 'capistrano-rails', '~> 1.1.0'
gem 'capistrano-rails-console'
# integrate bundler with capistrano
gem 'capistrano-bundler'
# if you are using RBENV
gem 'capistrano-rbenv', "~> 2.0"
gem 'capistrano-sidekiq', github: 'seuros/capistrano-sidekiq'
gem 'capistrano-passenger'
gem 'utf8-cleaner'
gem "devise"
gem 'rollbar', '~> 1.5.3'
#gem 'bson', '~> 3.0'
# 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
gem 'thin'
# 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', '~> 2.0.0.beta4'
# Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
gem 'spring'
#new gems
gem "rspec-rails"
gem 'pry-rails'
gem 'pry-rescue'
# Access an IRB console on exception pages or by using <%= console %> in views
gem 'better_errors'
#gem 'mongoid-rspec', '~> 2.1.0'
end

Answer 1

It looks like the error has appeared again in less-rails. The error is not related to Mongoid at all. I started getting the error a couple of days ago on my development server. I decided to check Github to see if an issue was reported about this. I found the following issue addressing the error.
https://github.com/metaskills/less-rails/pull/112
I changed my Gemfile to the following as provided on the link which corrected the error.
gem "less-rails", :git => 'https://github.com/suzan2go/less-rails.git', :branch => 'fix-nomethoderror-for-sprockets3'
As of now this solution has not been applied to the gem. I guess it will be soon.

Answer 2

Most important thing to look for while upgrading to Mongoid 5 is mongoid.yml
From:
<%= rails_env %>:
sessions:
default:
database: <%= mongodb_database %>
hosts:
- <%= mongodb_staging_host %>
username: <%= mongodb_user %>
password: <%= mongodb_password %>
options:
read: :primary_preferred
To:
<%= rails_env %>:
clients:
default:
database: <%= mongodb_database %>
hosts:
- <%= mongodb_staging_host %>
options:
user: <%= mongodb_user %>
password: <%= mongodb_password %>
read:
mode: :primary_preferred
Change sessions to clients
Move username (will become user) and password into options
change read mode as shown, if you have any.
Bottomline is to check mongoid.yml thoroughly while comparing with this page

Answer 3

Here how I fixed this issue
Removed from Gemfile gem "less-rails"
From application.rb removed require "mongo"
In mongoid.yml changed sessions: to clients: