Transfer MongoDB collection to SQL

mongo_to_sql.rb

# Taken from a rails app using mongoid and activrecord at the same time.
def transfer_mongo_to_sql(collection_name, fields=[])
  ic = Mongoid.default_session.collections.select { |c| c.name==collection_name }.first
  total_items = ic.find.count
  per_batch = 1000
  0.step(total_items, per_batch) do |offset|
    to_fields = fields.map { |c| c[:to_name] }.join(', ')
    insert_raw_sql = "INSERT INTO #{collection_name} (#{to_fields}) VALUES "

    before = Time.now
    vars = []
    raw_inserts = []
    ic.find.limit(per_batch).skip(offset).each do |item|
      raw_inserts << "(#{['?'].cycle(fields.length).to_a.join(', ')})"
      fields.each do |field|
        field_value = nil
        if field[:method].blank?
          field_value = item[field[:name]]
        else
          field_value = item[field[:name]].send(field[:method])
        end
        # if we have a default value set and our parsed field_value is blank
        if field[:default] and field_value.blank?
          field_value = field[:default]
        end
        vars << field_value
      end
    end
    insert_raw_sql = insert_raw_sql + raw_inserts.join(', ')
    sanitized_sql = ActiveRecord::Base.send(:sanitize_sql_array, [insert_raw_sql, *vars])
    puts "nr items: #{raw_inserts.length}"
    ActiveRecord::Base.connection.execute(sanitized_sql)
    time = Time.now - before
    puts "Job finished in #{time} secs"
  end
end