Rails 6 pass param in render :new after failed save - ruby-on-rails

Question

I have a standard create method in the controller responsible for creating a new Transaction record. The Transaction record has a mandatory transaction_type field which I hide in the view and automatically assigning it a value by passing it a params[:filter] so I have one _form for both withdrawal and deposit transactions, like below:
#index.html.erb
<%= link_to 'Add funds', new_transaction_path(filter: 'deposit') %>
<%= link_to 'Withdraw Funds', new_transaction_path(filter: 'withdrawal') %>
#new.html.erb
<%= render 'form', transaction: #transaction, transaction_type: params[:filter] %>
#_form.html.erb
<%= simple_form_for #transaction do |f| %>
<%= f.error_notification %>
<%= f.error_notification message: f.object.errors[:base].to_sentence if f.object.errors[:base].present? %>
<%= f.text_field :transaction_type, value: transaction_type, type: "hidden" %>
<%= f.input :amount, placeholder: 'Amount', label: false %>
<%= f.button :submit, 'Submit' %>
<% end %>
If for some reason the validation fails, to properly display the errors, the :new view will be rendered. Unfortunately, in this case, if the user fills out the entire form again (after first failed), the record will not be created because params[:filter] was not passed. Is there any way to pass original params[:filter] directly to the view?
#controller
# POST /transactions
def create
#transaction = wallet.transactions.new(transaction_params)
if #transaction.save
redirect_to :index, notice: 'Transaction was successfully created.'
else
render :new
end
end

Answer 1

While I understand the aspect of reusing the view code you really should consider creating separate routes and controllers and solving the code duplication issues by using inheritance and locals instead of by sneaking along a hidden parameter.
resources :deposits, :withdrawls, only: [:new, :create]
class TransactionsController < ApplicationController
helper_method :create_transaction_path
def new
#transaction = Transaction.new
render 'transactions/new'
end
def create
#transaction = Transaction.new(transaction_params) do |t|
t.transaction_type = transaction_type
end
if #transaction.save
yield #transaction if block_given?
success_response
else
yield #transaction if block_given?
failure_response
end
end
private
def transaction_type
controller_name.singularize
end
def create_transaction_path
polymorphic_path(controller_name)
end
def transaction_params
params.require(:transaction)
.permit(:foo, :bar, :baz)
end
def success_response
redirect_to transactions_path,
notice: 'Transaction was successfully created.'
end
def failure_response
render 'transactions/new'
end
end
class DepositsController < TransactionsController
# POST /deposits
def create
super do |transaction|
# do something just when making a deposit
end
end
end
class WithdrawlsController < TransactionsController
# POST /withdrawls
def create
super do |transaction|
# do something just when making a withdrawl
end
end
end
# app/views/transactions/_form.html.erb
<%= simple_form_for transaction, url: create_transaction_path do |f| %>
<%= f.error_notification %>
<%= f.error_notification message: f.object.errors[:base].to_sentence if f.object.errors[:base].present? %>
<%= f.input :amount %> # Do not use placeholders instead of labels
<%= f.button :submit, 'Submit' %>
<% end %>
<%= link_to 'Add funds', new_deposit_path %>
<%= link_to 'Withdraw Funds', new_withdrawl_path %>
Why?
Because it gives you endpoints that do a single job and it also gives you the obvious structure for your code when the requirements diverge as they most certainly will.

Answer 2

Stashing the value as a hidden field in the form is the right idea, but you're using two different parameter names for the same thing.
Your link_to call passes the transaction type as filter:
<%= link_to 'Add funds', new_transaction_path(filter: 'deposit') %>
In your form, you are putting it in a hidden field called :transaction_type. Thus, when the form submits the value now goes to your controller in params[:transaction_type]. One simple fix is to change the name of your hidden field:
<%= hidden_field_tag :filter, params[:filter] %>