gem 'delta', github: 'konukhov/delta'

class Order < ActiveRecord::Base
  has_many :line_items
  has_many :items, through: :line_items

  track_deltas :address, :promo_code, :items
end

order = Order.create(address: "Some address", items: [item1])
order.update address1: "New address", promo_code: "promo"
order.items << item2

[
  {
    id: 1,
    user: <User:...>, # if updated from controller with current_user
    model: <Order:...>,
    object: { # There's always an array
      [{
        name: "address",
        action: "C", # "C" == change, "A" == add, "R" == remove
        timestamp: 1441335511,
        object: "New address"
      },
      {
        name: "promo_code",
        action: "C",
        timestamp: 1441335511,
        object: "promo"
      }]
    }
  },
  {
    id: 2,
    user: <User:...>,
    model: <Order:...>,
    object: {
      [{
        name: "items",
        action: "A"
        timestamp: 1441335543,
        object: { "id" => 1 } # associations are serialized like this by default
      }]
    }
  }
]

class Company < ActiveRecord::Base
  has_many :employments
  has_many :employees, through: :employments
  belongs_to :user

  track_deltas_on :name

  track_deltas_on :employments, serialize: [:position, :salary],
                                notify: true, # Track association changes.
                                              # Could also be an association
                                              # name, like `notify: :company`.
                                only: []      # `only` option means which
                                              # actions (add, remove for
                                              # associations) to track.

  track_deltas_on :employees,   serialize: [:first_name, :last_name]
end

company = Company.create(name: "My company")
company.employees << employee # Created delta with serialized employee.

Employment
  .where(company: company, employee: employee)
  .first
  .update(position: "Engineer") # Created delta for company with serialized employemnt.