Select does not show errors from Rails' belongs_to presence validation about govuk-formbuilder

[peteryates]

Thank you for reporting this @sfnelson. As it's a problem that'd manifest in the same way on a vanilla Rails app I think we might be best just adding a cautionary paragraph to the guide. It feels like suggesting the disabling of the setting and requiring validations to be explicitly added is the way to go.

[peteryates]

Thinking about it, this problem will also happen in #govuk_collection_radio_buttons. Maybe the suggestion should go in the setup guide or README.

[sfnelson]

@peteryates thanks for the reply. That seems like a pragmatic approach at present. It would be good to have some guidance for users in how to resolve this disagreement between ActiveRecord's needs and ActiveView's needs – personally, I would to see the example at https://govuk-form-builder.netlify.app/form-elements/select/#select-field-with-a-label-and-hint to use an ActiveRecord association similar to https://guides.rubyonrails.org/v7.0/form_helpers.html#the-collection-select-helper

It seems like this is the way Rails has implemented belongs_to since at least Rails 4, so it's unlikely they are going to accept a change (e.g. recently proposed and closed rails/rails#46839). Would you consider a PR that uses reflection to identify associations and merges errors from association and association_id? This could be useful for more than just presence validations. Another approach to consider is rails/rails#46564

[sfnelson]

... As it's a problem that'd manifest in the same way on a vanilla Rails app ...

Vanilla Rails doesn't provide helpers for showing errors next to fields so the problem is not equivalent, e.g. this might be typical for a vanilla rails form:

<%= f.label :department_id %>
<% f.object.errors.messages_for(:department) do |message| %>
  <%= tag.div message class: "error" %>
<% end %>
<%= f.collection_select :department_id, departments, :id, :name %>

As an aside, there's a rubocop to flag and remove presence validations on belongs_to associations: https://docs.rubocop.org/rubocop-rails/cops_rails.html#railsredundantpresencevalidationonbelongsto

[peteryates]

Yeah, sorry I meant the underlying problem of the automatic validation being applied to the relationship but the form having to refer to the attribute name.

Would you consider a PR that uses reflection to identify associations and merges errors from association and association_id?

I think it could work but I suspect there's a fair bit of complexity involved. It might make sense for it to be implemented via an error summary presenter as that would allow people to opt in.

I'd still probably go with defining the validations explicitly, I'll create a PR with some amendments to the guide/README in the meantime.