gistfile1.rb

require 'json'

UNITS = {
  'unit_a' => 'kg.25',
  'unit_b' => 'kg.15',
  # ...
  'unit_c' => 'kg.5',
}

STORE = {
  'department_a' => {
    'cemento' => {
      price: 4.0,
      unit: UNITS['unit_a']
    },
    'cemento_bianco' => {
      price: 12.0
      unit: UNITS['unit_a']
    }
  },
  # ...
  'department_b' => {
    # ...
  }
}

def load_or_create_database(filename:)
  if File.exists?(filename)
    JSON.parse(File.read(filename))
  else
    quantities = {}
    STORE.each do |department, products|
      quantities[department] ||= {}
      products.each do |product_name|
        quantities[department][product_name] = 100
      end
    end
    File.write(filename, quantities.to_json)
    quantities
  end  
end

quantities = load_or_create_database(filename: 'store.json')

def product_price(department:, product_name:, requested_quantity:)
  amount = STORE[department][product_name][:price] * requested_quantity
  "$ #{amount}"
end

product_price(department: 'department_a', product_name: 'cemento_bianco', requested_quantity: 23) # => "$ 276"