transactions.ex

defmodule Transaction do
  defstruct [:merchant_id, :financial_context_id]
end

defmodule Transactions do
  def process do
    transactions = [
      %Transaction{merchant_id: "12345", financial_context_id: "1234"},
      %Transaction{merchant_id: "12345", financial_context_id: "1234"},
      %Transaction{merchant_id: "12345", financial_context_id: "1234"},
      %Transaction{merchant_id: "12345", financial_context_id: "1234"}
    ]

    %{
      merchant_ids: transactions |> Enum.map(&(&1.merchant_id)),
      financial_context_id: transactions |> Enum.map(&(&1.financial_context_id)),
      page_ids: transactions |> Enum.map(&get_page_id/1)
    }
  end

  def get_page_id(_transaction) do
    1
  end
end

Transactions.process |> IO.inspect