ActionView 'form cannot contain nil' after rails 5 scaffold generation

[kallisti5]

Steps to reproduce

rails g scaffold user email:uniq password:digest
rails g scaffold image --api name:string version:string cost:float public:bool memory:integer
rails g scaffold container --api name:string image_id:integer user_id:integer memory:integer

Expected behavior

Not error on image#new

Actual behavior

_form.html.erb where line #1 raised:

First argument in form cannot contain nil or be empty
Extracted source (around line #1):

<%= form_for(container) do |f| %>
  <% if container.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(container.errors.count, "error") %> prohibited this container from being saved:</h2>

      <ul>

Trace of template inclusion: app/views/containers/new.html.erb

System configuration

Rails version: Rails 5.0.0.beta3
Ruby version: 2.3.0p0

[kallisti5]

_A better trace_

ActionView::Template::Error (First argument in form cannot contain nil or be empty):
    1: <%= form_for(container) do |f| %>
    2:   <% if container.errors.any? %>
    3:     <div id="error_explanation">
    4:       <h2><%= pluralize(container.errors.count, "error") %> prohibited this container from being saved:</h2>

app/views/containers/_form.html.erb:1:in `_app_views_containers__form_html_erb___4094473587655993856_27833260'
app/views/containers/new.html.erb:3:in `_app_views_containers_new_html_erb__3949885752264488795_27160180'
  Rendered XXX/.gem/ruby/2.3.0/gems/actionpack-5.0.0.beta3/lib/action_dispatch/middleware/templates/rescues/_source.html.erb (6.2ms)
  Rendered XXX/.gem/ruby/2.3.0/gems/actionpack-5.0.0.beta3/lib/action_dispatch/middleware/templates/rescues/_trace.html.erb (2.3ms)
  Rendered XXX/.gem/ruby/2.3.0/gems/actionpack-5.0.0.beta3/lib/action_dispatch/middleware/templates/rescues/_request_and_response.html.erb (0.6ms)
  Rendered XXX/.gem/ruby/2.3.0/gems/actionpack-5.0.0.beta3/lib/action_dispatch/middleware/templates/rescues/template_error.html.erb within rescues/layout (25.7ms)
  Rendered /usr/lib/ruby/gems/2.3.0/gems/web-console-3.1.1/lib/web_console/templates/_markup.html.erb (0.2ms)
  Rendered /usr/lib/ruby/gems/2.3.0/gems/web-console-3.1.1/lib/web_console/templates/_inner_console_markup.html.erb within layouts/inlined_string (0.3ms)
  Rendered /usr/lib/ruby/gems/2.3.0/gems/web-console-3.1.1/lib/web_console/templates/_prompt_box_markup.html.erb within layouts/inlined_string (0.2ms)
  Rendered /usr/lib/ruby/gems/2.3.0/gems/web-console-3.1.1/lib/web_console/templates/style.css.erb within layouts/inlined_string (0.3ms)
  Rendered /usr/lib/ruby/gems/2.3.0/gems/web-console-3.1.1/lib/web_console/templates/console.js.erb within layouts/javascript (20.3ms)
  Rendered /usr/lib/ruby/gems/2.3.0/gems/web-console-3.1.1/lib/web_console/templates/main.js.erb within layouts/javascript (0.2ms)
  Rendered /usr/lib/ruby/gems/2.3.0/gems/web-console-3.1.1/lib/web_console/templates/index.html.erb (31.7ms)
Started GET "/containers/new" for ::1 at 2016-04-15 21:47:34 -0500
Processing by ContainersController#new as HTML
  Rendered containers/_form.html.erb (2.5ms)
  Rendered containers/new.html.erb within layouts/application (4.2ms)
Completed 500 Internal Server Error in 8ms (ActiveRecord: 0.0ms)

[Edouard-chin]

Your error comes from the fact that new method was not created in your controller when running the generator thus the @container instance variable was not assigned. (This is the template used when scaffolding with --api option)

This error is not surprising as an API controller shouldn't have the need to have a new method, a POST request to the create method can directly be made with the desired arguments to create your model.

The confusing part is that the route new_container GET /containers/new and edit_container GET /containers/:id/edit should not be created as well as the associated views (edit, new, _form)
And more globally, should we even create any view (html.erb) when scaffolding with the --api option? We basically only need the jbuilder template.

[maclover7]

Agree with @Edouard-chin -- it sounds like your Rails application should be a normal Rails app, not a special "API" application.

Can you please provide a sample application that replicates the error you are seeing? The likely issue here is that container should be @container.

[prathamesh-sonpatki]

The confusing part is that the route new_container GET /containers/new and edit_container GET /containers/:id/edit should not be created as well as the associated views (edit, new, _form)

Because the code generating routes check for config.api_only and not to the command line options.

@spastorino Do we want to support --api with generators? IMO it's a sideeffect as of now.

[prathamesh-sonpatki]

Should we rely on config.generators.api_only for the generators instead of options.api? ?

[sgrif]

Allowing --api to generators definitely seems unintentional

[prathamesh-sonpatki]

r? @spastorino

[jmbejar]

I think that supporting the --api/--no-api option in the generators was never the original intention, as @prathamesh-sonpatki @sgrif commented above.

I would simply remove the api (that is a class_option in a few generators) and rely directly on config.generators.api_only, so scaffold generators rely on this centralized flag.

I'll test this approach and confirm it's a valid fix.

[rafaelfranca]

It is fine to have --api in the generators, the real problem is that the views are being generated when they shouldn't

[fxn]

@dhh 👍

[kallisti5]

+1. You all are correct. Looking back I get why the error happened. However it seems like views were created that really don't make sense in api mode (which leads to user confusion)

[jeremy]

Looks like this isn't blocking release candidate, though we would like to address it for final release. Removing milestone.

[sgrif]

@jeremy Unsure what that means? If it'd be blocking the final release, presumably it should be blocking the release candidate.

[jeremy]

@sgrif Means it's desirable for 5.0.0 release, but it's a high flux area without sufficient "real app" validation, so it's reasonable to address in 5.0.1 or 5.0.x along with the rest of the issues that 5.0.0 flushes out. Added 5.0.1 milestone to communicate this "if we can make 5.0.0, great, else 5.0.1" designation.

[skipkayhil]

Fixed in #27604

On main (note that erb is empty):

$ rails g scaffold image --api name:string version:string cost:float public:bool memory:integer
      invoke  active_record
      create    db/migrate/20231029201748_create_images.rb
      create    app/models/image.rb
      invoke    test_unit
      create      test/models/image_test.rb
      create      test/fixtures/images.yml
      invoke  resource_route
       route    resources :images
      invoke  scaffold_controller
      create    app/controllers/images_controller.rb
      invoke    erb
      invoke    resource_route
      invoke    test_unit
      create      test/controllers/images_controller_test.rb
      invoke    helper
      create      app/helpers/images_helper.rb
      invoke      test_unit