ActiveAdmin calculations - ruby

Question

I want to implement some calculation to my admin interface, so i have a product resource, at this resource you see a list of services that I do, such as airbrushing, price on application considered as, for example (1 $ per 1 square cm).
How i can better realize this idea?
I would like to see when a user pushing a button "New Product" it was a field where he writes the number of square centimeters and on the basis of these dimensions, it automatically render the required amount in the currency.
Rails 4.1.0
ActiveAdmin 1.0.0
ruby 2.1
Just now you can only type a fixed price, like fixed price for 1 product/service.
app/admin/product.rb
ActiveAdmin.register Product, { :sort_order => :name_asc } do
# Scopes
scope :all, :default => true
scope :available do |products|
products.where("available < ?", Date.today)
end
scope :drafts do |products|
products.where("available > ?", Date.today)
end
scope :featured_products do |products|
products.where(:featured => true)
end
# Permitted parameters
permit_params :article_id, :title, :description, :price, :featured, :available, :image_file_name
# Displayed columns
index do
selectable_column
column :article, :sortable => :article
column :title, :sortable => :title
column :description
# Currency helper
column :price, :sortable => :price do |cur|
number_to_currency cur.price, locale: :ru
end
column :featured
column :available
# column :image_file_name
actions
end
# Product details
show do
panel "Product Details" do
attributes_table_for product do
row("Article") { link_to product.article }
row("Title") { product.title }
row("Description") { product.description }
row("Price") { product.price }
row("Featured") { product.featured }
row("Available on") { product.available }
row("Image") { image_tag("products/" + product.image_file_name) }
end
end
end
# Filters
filter :article, :as => :select
filter :title, :as => :select # :check_boxes (for checkboxes)
filter :price, :as => :select
filter :available, :as => :select
filter :featured, :as => :check_boxes
end
app/models/product.rb
class Product < ActiveRecord::Base
# Relationship
belongs_to :article
# Named Scopes
scope :available, lambda{ where("available < ?", Date.today) }
scope :drafts, lambda{ where("available > ?", Date.today) }
# Validations
validates :article, :title, :description, :price, :available, :presence => true
validates :featured, :inclusion => { :in => [true, false] }
end
app/models/article.rb
class Article < ActiveRecord::Base
# Relationship
has_many :products, :dependent => :delete_all
# Validations
validates :title, :description, :presence => true
# Define for display a article for products as article code
def to_s
"#{title}"
end
end