Why not use `params.fetch("transactions_log_expense")? If you used `params["transactions_log_expense"]`, and `params["transactions_log_expense"]` returned `nil`, then

```ruby
...
def create_expense
  event.expenses.create(expense_params)
end
...
```
would try to create `event.expenses` with a `transaction_log_expense` of `nil`.  Using `fetch` would ensure that the error occurs earlier (in `#initialize`) rather than later (in `#create_expense`).