A monkey patch for detecing nil IDs.

mongoid_collection.rb

require 'mongoid/collection'

module Mongoid
  class Collection
    alias_method :_update, :update

    def update(selector, document, options = {})
      document.deep_fetch("_id").each do |value|
        if value.nil?
          Rails.logger.error "Invalid nil _id in #{document}."
          raise "Invalid nil _id in #{document}."
        end
      end

      _update selector, document, options
    end
  end
end