After export all the record from MongoDB, use this to build relation in all records. You need to handle many to many relations by yourself

relation_builder.rb

# frozen_string_literal: true

require 'colorize'

module RelationBuilder
  attr_reader :constants

  def initialize(constants = [])
    if constants.present?
      @constants = constants
    else
      Rails.application.eager_load!
      @constants = ActiveRecord::Base.descendants.reject { |constant| constant.to_s.include?('::') }
    end
  end

  def build!(polymorphic_associations = [], rename_associations = {})
    constants.each do |constant|
      build_relation constant, polymorphic_associations, rename_associations
    end
  end

  def build_relations(class_name, polymorphic_associations = [], rename_associations = {})
    uuids = class_name.attribute_names.select { |name| name.end_with? '_uuid' }

    unless uuids.present?
      STDOUT.puts "#{class_name.to_s.yellow}: skipped"
      STDOUT.puts
      return
    end

    reset_callbacks class_name

    RelationBuilder.profiling do
      models_count = class_name.count
      class_name.unscoped.all.each_with_index do |model, index|
        update_params = uuids.map do |uuid|
          original_association_name = uuid[0...-5]

          association_model = association_model(
            original_association_name,
            model[uuid],
            polymorphic_associations,
            rename_associations
          )

          [original_association_name.to_s, association_model]
        end.compact

        begin
          Hash[update_params].each do |key, value|
            model.send "#{key}=", value
          end
          model.save! validate: false
        rescue Exception => e
          STDERR.puts e
          raise e
        end

        STDOUT.puts "#{index}/#{models_count}\n" if (counter % 1000).zero?
      end
    end
  end

  def association_model(original_name, uuid, polymorphics, rename_associations)
    association_name = rename_associations[original_name.to_sym] || original_name
    association_name = model["#{original_name}_type".to_sym] if polymorphics.include? original_name.to_sym

    return unless association_name.present?
    association_constant = association_name.to_s.camelcase.constantize
    association_constant.find_by uuid: uuid
  end

  def reset_callbacks(constant)
    %i(create save update).each do |callback|
      constant.reset_callbacks callback
    end
  end

  def self.profiling
    started_at = Time.now

    yield if block_given?

    seconds = Time.now.to_i - started_at.to_i
    STDOUT.puts "Completed: #{seconds.to_s.green} seconds\n"
    STDOUT.puts "\n"
  end
end

builder = RelationBuilder.new([Post, Comment])
builder.build!