Export MongoDB to csv files and import them into MySQL with mysqldump

database_transformer.rb

module DatabaseTransformer
  def self.import(collection, fields, columns)
    mongo_export = <<-EOF
    mongoexport --host #{hostname} --username #{username} \
                 --password #{password} \
                 --authenticationDatabase #{auth_source} \
                 --db #{database} --collection #{collection} \
                 --type=csv \
                 --fields #{fields.join(',')} \
                 --out #{collection}.csv
    EOF
    STDOUT.puts shell

    Dir.mktmpdir do |tmpdir|
      tmpdir = File.expand_path tmpdir
      Dir.chdir(tmpdir) do
        `#{mongo_export}`
        csv_file = "#{collection}.csv"
        connection = ActiveRecord::Base.connection
        connection.execute "set @@sql_mode='no_engine_substitution';"
        sql = "LOAD DATA LOCAL INFILE '#{tmpdir}/#{csv_file}' INTO TABLE #{collection} FIELDS TERMINATED BY ',' LINES TERMINATED BY '\n' IGNORE 1 LINES (#{columns.join(',')});"
        connection.execute sql
      end
    end
  end
end

DatabaseTransformer.import(:posts, [:title, :content], [:title, :content])