Converting Serialized YAML to Serialized JSON in Rails models

model_json.rb

class Medium < ActiveRecord::Base
  serialize :payload, JSON
end

model_yaml.rb

class Medium < ActiveRecord::Base
  serialize :payload
end

rake_task_code.rb

# create a facade model to avoid validations and other shenanigans
faux_medium = Class.new ActiveRecord::Base
faux_medium.table_name = "media"

# only loading the id and the payload helps avoid certain problems and speeds up the conversion
faux_medium.select([:id, :payload]).find_in_batches do |media|
  media.each do |medium|
    begin

      # manually encode to JSON because of the facade class
      medium.payload = YAML.load(medium.send(:attribute, :payload)).to_json
      medium.save!

    rescue Exception => e
      puts "Could not fix medium #{medium.id} for #{e.message}"
    end
  end
end

test_parsers.rb

hash = { "A" => "B", "C" => "D", "E" => "F", "G" => "H", "I" => { "J" => "K", "L" => "M", "N" => "O", "P" => "Q", "R" => "S" } }
yaml = hash.to_yaml
json = hash.to_json

yaml_time = Benchmark.realtime do
  1000.times do
    YAML.load yaml
  end
end # => 5.85665

json_time = Benchmark.realtime do
  1000.times do
    JSON.load json
  end
end # => 0.012307

Why use medium.send(:attribute, :payload) while not use medium.payload directly? My execution raises a error which said "undefined method `asstribute` for the class faux_medium".