routes.rb

resources :sellers, only: [ :index, :show ] do
    resources :contracts, shallow: true
end

routes_output.txt

             Prefix Verb   URI Pattern                                 Controller#Action
   seller_contracts GET    /sellers/:seller_id/contracts(.:format)     contracts#index
                    POST   /sellers/:seller_id/contracts(.:format)     contracts#create
new_seller_contract GET    /sellers/:seller_id/contracts/new(.:format) contracts#new
      edit_contract GET    /contracts/:id/edit(.:format)               contracts#edit
           contract GET    /contracts/:id(.:format)                    contracts#show
                    PATCH  /contracts/:id(.:format)                    contracts#update
                    PUT    /contracts/:id(.:format)                    contracts#update
                    DELETE /contracts/:id(.:format)                    contracts#destroy
            sellers GET    /sellers(.:format)                          sellers#index
             seller GET    /sellers/:id(.:format)                      sellers#show