csv_converter_example.rb

#
# Example of how to convert CSV data on read, in this case parsing
# some JSON in a CSV.
#
require 'csv'
require 'json'

csv = <<EOT
id,json_stuff
1,"{""foo"":""bar""}"
EOT

CSV.parse(csv, headers: true) do |row|
  puts row["json_stuff"].class # => String
end

# You can create a lambda that takes one or two arguments to convert values
# on read.
#
# field is the raw value, field_info is a CSV::FieldInfo struct with the
# members index, line and header
json_parser_converter = lambda do |field, field_info|
  # Every single field in the CSV gets passed to this block, so you need to
  # the field content or catch exceptions if the field data can't be converted.
  if field_info.header == "json_stuff" && field != ""
    JSON.parse(field)
  else
    field
  end
end

CSV.parse(csv, headers: true, converters: [json_parser_converter]) do |row|
  puts row["json_stuff"].class # => Hash
end


# The CSV lib also has an internal global hash of converters
CSV::Converters[:json_parser_converter] = json_parser_converter

# Note the converter is specified as a symbol here.
CSV.parse(csv, headers: true, converters: [:json_parser_converter]) do |row|
  puts row["json_stuff"].class # => Hash
end

# You can use :all to use all converters, but it will only use all the built in
# converters, not any custom ones you have added.
CSV.parse(csv, headers: true, converters: [:all]) do |row|
  puts row["json_stuff"].class # => String
end