A read-only frontend API abstraction applicable to any application.

frontend_api.rb

# frozen_string_literal: true

module Api
  module V1
    # Controller to consume read-only data to be used on client's frontend
    class FrontEndController < ActionController::API
      prepend_before_action :set_root_resource
      before_action :set_object, except: %i[index schema]
      append_before_action :set_nested_resource, only: %i[nested_index]
      append_before_action :set_records, only: %i[index nested_index]
      append_before_action :set_schema, only: %i[schema]

      include Orderable

      # GET /:resource
      # GET /:resource.json
      def index
        render json: records_json
      end

      # GET /:resource/1
      # GET /:resource/1.json
      def show
        render json: @object.to_json(include: parsed_include)
      end

      # GET /:resource/1/:nested_resource
      # GET /:resource/1/:nested_resource.json
      def nested_index
        render json: records_json
      end

      # OPTIONS /:resource
      # OPTIONS /:resource.json
      # OPTIONS /:resource/1/:nested_resource
      # OPTIONS /:resource/1/:nested_resource.json
      def schema
        render json: @schemated.to_json
      end

      private

      # Common setup to stablish which model is the resource of this request
      def set_root_resource
        @root_resource = params[:resource].classify.constantize
      end

      # Common setup to stablish which object this request is querying
      def set_object
        id = params[:id]
        @object = resource.friendly.find(id)
      rescue NoMethodError
        @object = resource.find(id)
      end

      # Setup to stablish the nested model to be queried
      def set_nested_resource
        @nested_resource = @object.send(params[:nested])
      end

      # Used to setup the resource's schema
      def set_schema
        @schemated = {}
        raise ActionController::BadRequest('Invalid resource') unless resource.present?
        resource.columns_hash.each { |key, value| @schemated[key] = value.type }
      end

      # Used to setup the records from the selected resource
      # that are going to be rendered
      def set_records
        @records = resource.order(ordering_params(params))
                            .ransack(parsed_query).result
      end

      # Used to avoid errors in JSON parsing
      def parsed_query
        JSON.parse(params[:q])
      rescue JSON::ParserError, TypeError
        {}
      end

      # Used to avoid errors in included associations parsing
      def parsed_include
        params[:include].split(',')
      rescue NoMethodError
        []
      end

      # Parsing of `@records` variable to paginated JSON
      def records_json
        @records.paginate(page: params[:page], per_page: params[:per_page])
                .to_json(include: parsed_include)
      end

      # Reutrns root_resource if nested_resource is not set
      def resource
        @nested_resource || @root_resource
      end
    end
  end
end

orderable.rb

# frozen_string_literal: true

# This concern is used to provide abstract ordering based on `params[:sort]`
module Orderable
  extend ActiveSupport::Concern
  SORT_ORDER = { '+' => :asc, '-' => :desc }.freeze

  # A list of the param names that can be used for ordering the model list
  def ordering_params(params)
    # For example it retrieves a list of orders in descending order of total_value.
    # Within a specific total_value, older orders are ordered first
    #
    # GET /orders?sort=-total_value,created_at
    # ordering_params(params) # => { total_value: :desc, created_at: :asc }
    #
    # Usage:
    # Order.order(ordering_params(params))
    ordering = {}
    params[:sort].try(:split, ',').try(:each) do |attr|
      attr = parse_attr attr
      model = controller_name.titlecase.singularize.constantize
      if model.attribute_names.include?(attr)
        ordering[attr] = SORT_ORDER[parse_sign attr]
      end
    end
    ordering
  end

  private

  # Parsing of attributes to avoid empty starts in case browser passes "+" as " "
  def parse_attr(attr)
    '+' + attr.gsub(/^\ (.*)/, '\1') if attr.starts_with?(' ')
  end

  # Ordering sign parse, which separates
  def parse_sign(attr)
    attr =~ /\A[+-]/ ? attr.slice!(0) : '+'
  end
end

routes.rb

get '/:resource/', to: 'front_end#index', via: :get
get '/:resource/:id', to: 'front_end#show', via: :get
get '/:resource/:id/:nested/', to: 'front_end#nested_index', via: :get
match '/:resource/', to: 'front_end#schema', via: :options
match '/:resource/:id/:nested/', to: 'front_end#schema', via: :options

Append and prepend are there to assure the expected execution order, is it a little overkilled?

For what I can see you tried to use @root_resource and @nested_resource to differentiate theirs uses.
But, I thought it a little confusing at first sight. First because set_resource sets @resource (and @root_resource), but set_nested also sets @resource.

What do you think about using only @resource and changing the method names to set_root_resource and set_nested_resource ?

Ooops... I think my suggestion don't apply. You set object from root_resource, then set nested_resource, from object.

Look at this diff. Perhaps a better approach (than my last suggestion).
https://gist.github.com/abinoam/762802f4f48e9d1aa98d48940b2fd724/revisions#diff-dd7bb1d0357534ad4ec0f31681fbe64b