use Mongoid 3 and delete all collections before :each spec

spec_helper.rb

# Mongoid 3 uses Moped as MongoDB driver, not mongo-ruby-driver.
# Thus we should replace some codes.
# Change your spec/spec_helper.rb like this code below:

RSpec.configure do |config|
  config.before(:each) do
    Mongoid.session(:default).collections.each do |collection|
      # Each Moped::Collection instance is picked up,
      # and drop the collection.
      collection.drop
    end

    # # If you are a Mongoid 2 user, choose this code below:
    # Mongoid.database.collections.each do |collection|
    #   unless collection.name =~ /^system\./
    #     collection.remove
    #   end
    # end
  end
end