Use Commanded.Aggregate.Multi to generate multiple events from a single command.

This can be useful when you want to emit multiple events that depend upon the aggregate state being updated.

Example

In the example below, money is withdrawn from the bank account and the updated balance is used to check whether the account is overdrawn.

defmodule BankAccount do
  alias Commanded.Aggregate.Multi

  defstruct [:account_number, :state, balance: 0]

  def withdraw(
    %BankAccount{state: :active} = account,
    %WithdrawMoney{amount: amount})
    when is_number(amount) and amount > 0
  do
    account
    |> Multi.new()
    |> Multi.execute(&withdraw_money(&1, amount))
    |> Multi.execute(&check_balance/1)
  end

  defp withdraw_money(%BankAccount{account_number: account_number, balance: balance}, amount) do
    %MoneyWithdrawn{
      account_number: account_number,
      amount: amount,
      balance: balance - amount
    }
  end

  defp check_balance(%BankAccount{account_number: account_number, balance: balance})
    when balance < 0
  do
    %AccountOverdrawn{account_number: account_number, balance: balance}
  end
  defp check_balance(%BankAccount{}), do: []
end