validation.rb

module ProfileValidator
  def self.validate(profile, scentregroup_id)
    profile_schema.call(profile.merge(scentregroup_id: scentregroup_id))
  end

  def self.profile_schema
    required(:centre).filled
    required(:number_plates).each do
      schema do
        required(:plate).filled
        required(:registered_in).filled
        
        validate(plate_available: %i[plate]) do |plate|
          ### How can I use the centre attribute in here?
        end
      end
    end
  end
end

I'm not sure you can refer to attributes outside of the sub-schema like you're hoping to.

Perhaps simplest thing is to have a data mapper that you run before calling the schema, which would copy the centre attribute into each of the number_plates hashes? Then you could add required(:centre).filled inside that sub-schema and use the centre attribute inside your validate block.

I admit this is a bit awkward, but it'd get the job done here, at least.

I'd encourage you to file this as an issue on the GitHub repo, though. We're going to put our attention back onto dry-validation at some point before too long, and getting this as an issue here would at least mean we can consider this use case :)

Ah ok. I am a bit relieved that I was not missing something super obvious, thanks @timriley :)