Do not generate unused components contents in `app:update` task

[y-yagi]

Currently, app:update generates all contents regardless of the component using in application.

For example, even if not using Action Cable, app:update will generate a contents related to Action Cable. This is a little inconvenient.
This PR checks the existence of the component and does not generate unnecessary contents.
Can not check all options in this way. However, it will be able to prevent the generation of unnecessary files.

[rails-bot]

r? @matthewd

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[kaspth]

Didn't we fix this with the .railsrc file?

[y-yagi]

Currently, app:update task does not use .railsrc.
Also, since the user who executed rails new and the user who executes app:update are not necessarily the same, .railsrc can not be used as it is.

[matthewd]

@kaspth we talked about it.. I think there might be an unmerged PR?

IIRC I wasn't excited about adding an extra top-level file for such a rare consideration (plus the potential confusion if you've added/removed a component since the initial generate), so this does seem potentially interesting to me.

[y-yagi]

Probably I think that it is about #22790

[rafaelfranca]

Can we make the assumption that if ActiveRecord is defined we are loading Active Record? I don't think this is true since a gem can define this constant. Maybe we should check for the railties?

[y-yagi]

It makes sense. I updated to check railties.

[kaspth]

I dig this! Simple, clean. Got 2 things, then 👍

[kaspth]

Maybe we should condense these:

def generator_options
  options = { api: !!Rails.application.config.api_only, update: true }
  options[:skip_active_record] = !defined?(ActiveRecord::Railtie)
  options[:skip_action_mailer] = !defined?(ActionMailer::Railtie)
  options[:skip_action_cable]  = !defined?(ActionCable::Engine)
  options[:skip_sprockets]     = !defined?(Sprockets::Railtie)
  options[:skip_puma]          = !defined?(Puma)
  options
end

or even just make it a standard hash definition.

[kaspth]

Let's flesh this out a bit more. How about:

*   Skip unused components when running `bin/rails app:update`

    If the initial app generation skipped Action Cable, Active Record etc.,
    the update task honors those skips too.

[kaspth]

Right, #22790 was what I was thinking of — and I was sure we had merged that. But this is far simpler 👍

[y-yagi]

Thank you for review! I fixed.

[kaspth]

Nice!