How to Use Ash Calculations to Sum total of list items Without Hitting the Data Layer

Questions & Help Questions
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9

It was simpler than I thought! I followed the contract of extracting a calculation in its module and it worked.

# MyApp.Sales.Sale resource
  calculations do
    calculate :sub_total, :decimal, MyApp.Sales.Sale.Calculations.SubTotal
  end

Calculation Module

defmodule MyApp.Sales.Sale.Calculations.SubTotal do
  use Ash.Resource.Calculation

  def description, do: "Calculates the sub total of a sale."
  def public?, do: true

  @impl true
  def load(_query, opts, _context) do
    [line_items: [:line_total, :quantity, :price]]
  end

  @impl true
  def calculate(sales, _opts, _arguments) do
    Enum.map(sales, &calculate_sub_total/1)
  end

  defp calculate_sub_total(%{line_items: line_items}) do
    line_items
    |> Enum.map(& &1.line_total)
    |> Enum.reduce(Decimal.new(0), &Decimal.add/2)
  end
end

Now I am able to successful calculate subtotal without hitting the DB

sale_attrs = %{line_items: [%{line_total: 12}, %{line_total: 15}]}
Ash.calculate!(Zippiker.Sales.Sale, :sub_total, refs: sale_attrs)

# Decimal.new("27")