Template Error: missing partial bar/foo/_new

I want to display a root page which shows a turbo frame with a form for creating a new object. However the routing doesn't work as I want, because I get the above mentioned error.

routes.rb

namespace :cust do
  resources :customer_dates, only: [:index, :show, :create, :new]
end
root "cust/customer_bookings#index"

/views/cust/customer_bookings/index.html.erb

<div><%= render "main" %></div>

/views/cust/customer_bookings/_main.html.erb

<%= turbo_frame_tag @customer_date do %>
  <%= render new_cust_customer_date_path, customer_date: @customer_date %>
<% end %>

Error

> ActionView::Template::Error (Missing partial cust/customer_dates/_new with {:locale=>[:en], :formats=>[:html], :variants=>[], :handlers=>[:raw, :erb, :html, :builder, :ruby, :jbuilder]}.

And the line causing the error is the one in _main.html.erb with new_cust_customer_date_path.
In /views/cust/customer_dates folder I have the views new.html.erb and _form.html.erb

new_cust_customer_date_path is a url helper, you can use it with link_to, button_to, form_with url: etc. render works with your local files, which may or may not match the url path.

To clarify:

"localhost:3000/cust/customer_dates/new" # request from the browser
# bin/rails routes                       # will match one of the routes
namespace :cust do                       # `/cust` to `module Cust`
  resources :customer_dates,             # `/customer_dates` to `CustomerDatesController`
    only: [:index, :show, :create, :new] # `/new` to `def new`
end                                      # Cust::CustomerDatesController#new
def new                                  # controller action
  @customer_date = CustomerDate.new      # do some things
                                         # unless you render something, rails
  # render :new                          # will implicitly `render "#{action_name}"`
end                                      # in controllers, `render` defaults to
new.html.erb                             # rendering a template with layout:
                                         # render template: "cust/customer_dates/new", layout: "application"

Inside the views, render defaults to rendering a partial and no layout.

new_cust_customer_date_path #=> /cust/customer_dates/new

<%= render "/cust/customer_dates/new", customer_date: @customer_date %>
# which is the same as
<%= render partial: "/cust/customer_dates/new", locals: { customer_date: @customer_date } %>

# partial is the same as template, but will auto prefix `_` to the filename
<%= render template: "/cust/customer_dates/_new", locals: { customer_date: @customer_date } %>

Since you don't have _new.html.erb file, you get an error. If you need to render a form, you can do this:

<%= render "cust/customer_dates/form", customer_date: @customer_date %>
#                               ^
#                               form is a partial `_form`

# or render `new.html.erb` as a template (no underscore)
#                                         v
<%= render template: "cust/customer_dates/new", locals: { customer_date: @customer_date } %>

Note that "cust/customer_dates/new" is just a path relative to app/views.

https://guides.rubyonrails.org/layouts_and_rendering.html