mapper.rb

class Something
  def self.display_map
    [
      {
        key: :position,
        name: 'Number'
      },
      {
        key: :design_collection,
        name: 'Collection',
        relation: true,
        columns: [:name, :id]
      },
      {
        key: :design_style_details,
        relation: true,
        columns: [:id]
      }
    ]
  end
end

class Mapper
  def initialize(klass, scope: nil)
    @klass = klass
    @scope = scope&.to_sym || :all
  end

  def map_items
    @klass.send(@scope).map do |record|
      @klass.display_map.inject({obj: record}) do |hash, att|
        data = record.send(attr[:key])
        if (reflection = @klass.reflect_on_association(attr[:key]))
          if reflection.collection?
            data = data.map { |r| r.slice(att[:columns]) }
          else
            data = data.slice(att[:columns])
          end
        end
        hash.update(attr[:key] => { data: data, name: att[:name] })
      end
    end
  end
end

# and slice is implemented on client so here it is: (see https://github.com/ruby-hyperloop/hyper-mesh/issues/36)

module ActiveRecord
  module InstanceMethods
    def slice(methods)
      Hash[methods.map { |method| [method, send(method)] }]
    end
  end
end