Rails money gem and form builder

You can now edit monetized fields directly (money-rails 1.3.0):

# add migration
add_column :products, :price, :price_cents

# set monetize for this field inside the model
class Product
  monetize :price_cents
end

# inside form use .price instead of .price_cents method
f.text_field :price

See https://stackoverflow.com/a/30763084/46039

Given a migration as follows:

class CreateItems < ActiveRecord::Migration
  def self.up
    create_table :items do |t|
      t.integer :cents
      t.string :currency
      t.timestamps
    end
  end

  def self.down
    drop_table :items
  end
end

And a model as follows:

class Item < ActiveRecord::Base
  composed_of :amount,
    :class_name  => "Money",
    :mapping     => [%w(cents cents), %w(currency currency_as_string)],
    :constructor => Proc.new { |cents, currency| Money.new(cents || 0, currency || Money.default_currency) },
    :converter   => Proc.new { |value| value.respond_to?(:to_money) ? value.to_money : raise(ArgumentError, "Can't conver #{value.class} to Money") }
end

Then this form code should work perfectly (I just tested under Rails 3.0.3), properly displaying and saving the dollar amount every time you save/edit. (This is using the default scaffold update/create methods).

<%= form_for(@item) do |f| %>
  <div class="field">
    <%= f.label :amount %><br />
    <%= f.text_field :amount %>
  </div>
  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>

If you have multiple money fields in your table and you can't name them all "cents".

class CreateItems < ActiveRecord::Migration
  def self.up
    create_table :items do |t|
      t.integer :purchase_price_cents
      t.string :currency
      t.timestamps
    end
  end

  def self.down
    drop_table :items
  end
end

which would change your model to

class Item < ActiveRecord::Base

  composed_of :purchase_price,
    :class_name  => "Money",
    :mapping     => [%w(purchase_price_cents cents), %w(currency currency_as_string)],
    :constructor => Proc.new { |purchase_price_cents, currency| Money.new(purchase_price_cents || 0, currency || Money.default_currency) },
    :converter   => Proc.new { |value| value.respond_to?(:to_money) ? value.to_money : raise(ArgumentError, "Can't convert #{value.class} to Money") }

end