Question

Why can’t I save changes in an associated object saving a current object if attributes of the first one are represented by a value object?

For example, I have a simple e-commerce application. It use a Client model for manipulating clients

Client model

# db/migrate/<...>_create_clients.rb

class CreateClients < ActiveRecord::Migration
  def self.up
    create_table :clients do |t|
      t.string :name
    end
  end

  def self.down
    drop_table :clients
  end
end

# app/models/client.rb

Client < ActiveRecord::Base
  has_one :balance
end

and an associated Balance model for holding thier balances.

Balance model

# db/migrate/<...>_create_balances.rb

class CreateBalances < ActiveRecord::Migration
  def self.up
    create_table :balances do |t|
      t.integer    :amount
      t.string     :currency
      t.references :client
    end
  end

  def self.down
    drop_table :balances
  end
end

# app/models/balance.rb

class Balance < ActiveRecord::Base
  belongs_to :client

  composed_of :money,
    :mapping     =>
      [%w{amount cents}, %w{currency currency_as_string}],
    :constructor =>
      ->(amount, currency) { Money.new(amount || 0, currency || 'RUB') }
end

The Balance model uses a Money object from an external library called Money. The object represents model’s attributes amount and currency adding to the model useful methods for manipulating those attributes.

Gemfile

# Gemfile

gem 'money'

I have some seed data in seeds.rb.

Seeds

# db/seeds.rb

elena = Client.create(:name => 'Elena')
elena.build_balance.money = Money.new(0, 'RUB')
elena.save

When I try to change a balance of the client it isn't changed in spite of the save method of the current object returns true.

>> elena = Client.find_by_name('Elena')
=> #<Client id: 1, name: "Elena">

>> elena.balance
=> #<Balance id: 1, amount: 0, currency: "RUB", client_id: 1>

>> elena.balance.money
=> 0.00

>> elena.balance.money += Money.new(50000, 'RUB')
=> 500.00

>> elena.save
=> true

# log/development.log
# no changes

However, I can save the changes using the following two ways.

1.

>> elena = Client.find_by_name('Elena')
=> #<Client id: 1, name: "Elena">

>> balance = Balance.find(elena.id)
=> #<Balance id:1 , amount: 0, currency: "RUB", client_id: 1>

>> balance.money += Money.new(50000, 'RUB')
=> 500.00

>> balance.save
=> true

# log/development.log
# UPDATE "balances" SET "amount" = 50000 WHERE ("balances"."id" = 1)

2.

>> elena = Client.find_by_name('Elena')
=> #<Client id:1, name:"Elena">

>> elena.balance.money += Money.new(50000, 'RUB')
=> 500.00

>> elena.balance.save
=> true

# log/development.log
# UPDATE "balances" SET "amount" = 50000 WHERE ("balances"."id" = 1)

Despite I can save changes of a balance using the aforementioned ways I would like to know why I can't do it using the “traditional” <current_model>.save method.

Thanks.

Debian GNU/Linux 5.0.6;

Ruby 1.9.2;

Ruby on Rails 3.0.1;

Money 3.1.5.

Answer 1

It is accomplished by doing this:

# app/models/client.rb

Client < ActiveRecord::Base
  has_one :balance, :autosave => true
end

The problem with the association before is that when you were invoking elena.save, it was only looking to see if the Client object had changed. If you performed a check like this on the first example:

>> elena.changed?
=> false
>> elena.balance.changed?
=> true

If you don’t inform ActiveRecord to check the associated models it will be lazy and ignore changes when saving the parent model.