display_on? on the default :new action item raises NameError (v4.0.0.beta22)

[joker-777]

Summary

Calling display_on? on the default :new action item (the one AA registers
automatically for every resource, added in ActiveAdmin::Resource::ActionItems #add_default_new_action_item) raises a NameError because its if: lambda
references active_admin_config, which is not resolvable in the context the
lambda is evaluated from when triggered via display_on?.

Reproduction

ActiveAdmin.register SomeModel do
  config.action_items.delete_if do |item|
    item.name == :new && item.display_on?(:index)
  end
end

Booting a page for SomeModel (or anything that walks config.action_items
and calls display_on?) raises:

NameError: undefined local variable or method 'active_admin_config' for an instance of ActiveAdmin::ActionItem

Root cause (as far as we've traced it)

lib/active_admin/resource/action_items.rb registers the default :new item
with:

add_action_item :new, only: :index, if: -> { new_action_authorized?(active_admin_config.resource_class) } do
  ...
end

In 3.x, the equivalent visibility check ran inline, evaluated lazily at
render time in a view context where active_admin_config resolves. In
4.0.0.beta22, the check moved into a separate if: lambda that gets
evaluated by ActiveAdmin::OptionalDisplay#display_on? outside that view
context, where active_admin_config is no longer in scope.

Any resource-level code that calls display_on? on the default :new item
(a previously-working, documented pattern for conditionally hiding it) will
hit this.

Environment

• activeadmin 4.0.0.beta22 (gem, from github.com/activeadmin/activeadmin
  at tag v4.0.0.beta22)
• Ruby 3.4.9, Rails 7.2.3
• Reproduced independently of activeadmin_addons or any other gem — bare AA
  v4 core against a single custom resource registration.

What we did as a workaround

We dropped the display_on? probe and delete by name only, since the
default :new item is registered only: :index — for the default item,
name == :new && display_on?(:index) and name == :new are equivalent:

config.action_items.delete_if { |item| item.name == :new }

This sidesteps the lambda entirely but obviously doesn't fix the underlying
display_on?/if:-lambda scoping issue for anyone who needs to call
display_on? for a different reason.

Happy to provide more context or a minimal repro repo if useful.

[javierjulio]

@joker-777 thank you. What are you trying to do and why? The ActionItems module has a remove_action_item method which only deletes by name and that is consistent in both v3 and v4. Further, we don't document display_on? usage so from my perspective it's a helper method that is only used internally. It seems odd to run that removal using that method in that scope. If you are able to resolve for all cases, meaning how it's used currently and for your specific use case, I will accept a PR for it. Until then though I don't see this as an issue. Looks like I started to implement changes to this from #7085 which it's final form was in #8205.