Arel inside select list

gistfile1.rb

class Client
  has_many :sales, dependent: :restrict_with_error    
  
  # This is an example of how to use an Arel projection inside a select list.
  scope :with_sales_totals, -> {
    clients = arel_table
    sales = Sale.arel_table
      
    # this is will be agregated in the final query in the select list
    receivables_sum = sales
      .project(sales[:receivable].sum)
      .where(sales[:client_id].eq(clients[:id]))
      
    joins(sales: :sale_items)
      .select(
        'clients.*', 
        'SUM(qty*unit_price) AS sales_total', 
        receivables_sum.as('sales_receivable'))
      .group('clients.id')
  }
  
end