class CreateUsers < ActiveRecord::Migration
  def change
    # we create the User table as an example
    create_table :users do |t|
      # we create an attribute called "metadata" as an example
      t.string :metadata
      t.timestamps null: false
    end  
  end
end

class User < ActiveRecord::Base
  serialize :metadata, JSON

  # Alternatively, you can also serialize an attribute as a Hash or Array
  # serialize :metadata, Hash
  # serialize :metadata, Array
end

• http://apidock.com/rails/v3.1.0/ActiveRecord/Base/serialize/class