### route

  resources :flow_templates do
    resources :flow_details, shallow: true
  end

### Model

  class FlowTemplate < ActiveRecord::Base
    has_many :details, class_name: :FlowDetail, foreign_key: :template_id
  end

  class FlowDetail < ActiveRecord::Base
    belongs_to :template, class_name: :FlowTemplate, foreign_key: :template_id
  end


### Controller

  class FlowTemplatesController < ApplicationController
    inherit_resources
    actions :all, except: [:show]

    private
      def flow_template_params
        params.require(:flow_template).permit!
      end
  end

  class FlowDetailsController < ApplicationController
    inherit_resources
    belongs_to :FlowTemplate

    private
      def flow_detail_params
        params.require(:flow_detail).permit!
      end
  end

### error

  http://127.0.0.1:5000/flow_templates/1/flow_details/new
  Couldn't find FlowTemplate without an ID