application_controller_spec.rb

RSpec.describe ApplicationController do

  it_behaves_like "JsonRepresenterConcern"

end

json_representer_concern.rb

module JsonRepresenterConcern
  extend ActiveSupport::Concern

  included do
    private

    def represent(record)
      return nil if record.blank?
      klass_name = record.class.name

      "#{klass_name}Presenter".constantize.new(record)
    end

    def represent_collection(records)
      return [] if records.empty?
      klass_name = record.class.name

      records.map do |record|
        "#{klass_name}Presenter".constantize.new(record)
      end
    end
  end
end

json_representer_concern_shared_examples.rb

RSpec.shared_examples "JsonRepresenterConcern" do

  # let(:dummy) { Class.new } ## dummy.new.class.name is nil, I need some sort of class name

  # Pollutes namespace & opens Dummy that may be defined in other specs
  class Dummy
    include JsonRepresenterConcern
  end
  
  # let(:subject) { double } ## subject.class.name returns RSpec::Mocks::Double which is .. well it feels wrong to use that
  let(:subject) { Dummy.new }

  describe "#represent_collection" do
    it "wraps a record in a presenter" do
      representation = subject.send :represent_collection, []
      # Expect some stuff
    end
  end

  describe "#represent_collection" do
    it "maps a collection of records to records wrapped in a presenter" do
    end
  end
end