devise_token_auth with an existing Rails application

After rails g devise_token_auth:install User auth I had to change generated migration by

class DeviseTokenAuthUpdateUsers < ActiveRecord::Migration[5.1]
  def change
    add_column :users, :tokens, :json
  end
end

The User model, I left unchanged devise modules

  devise :database_authenticatable, :recoverable, :rememberable, :trackable, :validatable

and routes by

  namespace :api do
    scope :v1 do
      mount_devise_token_auth_for 'User', at: 'auth'
    end
  end

because exiting routes