export_csv_from_sql.rb

# Given the location of an SQL query file, this method will
# execute the query on the database, and print each line to 
# the supplied target file.
# This method relies on the MySQL adapter for the fetch_fields 
# method, but should not suffer in performance from pulling 
# large amounts of data into memory.

public

def export_csv_from_sql(sql_file,target_file)

  raise ArgumentError, "SQL file not found : #{sql_file}" unless File.exists?(sql_file)

  # Result of the SQL query, but does not put all data in memory
  data = ActiveRecord::Base.connection.execute(File.open(sql_file).read)

  # CSV header
  keys = data.fetch_fields.map(&:name)

  FasterCSV.open(target_file,'w') do |csv|
    csv << keys

    # This line fetches the data a row at a time from query result
    data.each { |row| csv << row }
  end
end