defmodule PlatformCore.Repo.Migrations.AddMarginContributionSettlementMisItems do
  use Ecto.Migration

  @moduledoc """
  Phase 1.5: Add margin_contribution to settlement_mis_items.

  margin_contribution = total_mdr_amount - total_interchange_fee

  This is Mercury's gross margin on a per-group basis:
  - total_mdr_amount: what Mercury charged the merchant
  - total_interchange_fee: what Mercury paid to the card scheme
  - margin_contribution: Mercury's gross margin (difference)

  Interchange does NOT reduce merchant payout — it is Mercury's internal cost.
  The margin column is purely for Finance team reporting and profitability analysis.
  """

  def change do
    alter table(:settlement_mis_items) do
      add :margin_contribution, :decimal, precision: 18, scale: 4, null: true,
        comment: "Mercury gross margin = total_mdr_amount - total_interchange_fee"
    end
  end
end