Rails: Getting the Parent Object of a Nested Resource

Question

I have a nested resource that belongs to many different models. For instance:

resources :users do
  resources :histories, only: [:show]
end

resources :publications do
  resources :histories, only: [:show]
end

resources :events do
  resources :histories, only: [:show]
end

In the HistoriesController, I want to find the parent object, though I'm having trouble thinking of a dry way to handle this. At the moment, the best I can come up with is:

if params[:user_id].present?
  @parent = User.find(params[:user_id])
elsif params[:publication_id].present?
  @parent = Publication.find(params[:publication_id])
elsif . . . .

I've got literally dozens of models I have to branch through in this way, which seems sloppy. Is there a better (perhaps baked-in) approach that I'm not considering?

Answer 1

The way I am doing this is adding the parent model class name as a default param in the route.

For the question example this should be something like:

resources :users, model_name: 'User' do
  resources :histories, only: [:show]
end

resources :publications, model_name: 'Publication' do
  resources :histories, only: [:show]
end

resources :events, model_name: 'Event' do
  resources :histories, only: [:show]
end

This will add the model name in the params hash.

Then in the controller/action you can get your parent model like:

params[:model_name].constantize # Gives you the model Class (eg. User)

and the foreign key like:

params[:model_name].foreign_key # Gives you column name (eg. user_id)

So you can do something like:

parent_class = params[:model_name].constantize
parent_foreing_key = params[:model_name].foreign_key

parent_object = parent_class.find(params[parent_foreing_key])

Answer 2

not really a solution but you can get away with

parent_klasses = %w[user publication comment]
if klass = parent_klasses.detect { |pk| params[:"#{pk}_id"].present? }
  @parent = klass.camelize.constantize.find params[:"#{klass}_id"]
end

if you are using a convention between your parameter name and your models

Answer 3

As an alternative to the accepted answer, you could use a dynamic route like this:

get ':item_controller/:item_id/histories/:id', to: 'histories#show'

This should then should allow you to access the parent class something like this in your histories_controller.rb

parent_controller = params[:item_controller]
parent_class = parent_controller.singularize.camelize.constantize
@parent = parent_class.find(params[:item_id])

You may be able to add a constraint on item_controller in the routes as well if you need to.