The better way to pass the foreign_key value to the Rails controller

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You may want to consider reading the Rails Guide on nested resources:

http://guides.rubyonrails.org/routing.html#nested-resources

In a nutshell:

routes.rb

resources :galleries do
  resources :pictures do
end
# Generates the routes: /galleries/:gallery_id/pictures

pictures_controller.rb 

def new
  @gallery = Gallery.find(params[:gallery_id])
  @picture = Picture.new
end
def create
  @gallery = Gallery.find(params[:gallery_id]) # gallery_id is passed in the URL
  @picture = @gallery.build(params[:picture])
  if @picture.save
  # success
  else
  # fail
  end
end

pictures/new.html.erb 

<%= form_for [@gallery, @picture] do |f| %>
  <div class="field"> 
    <%= f.hidden_field :gallery_id , :value=>params[:gallery_id] %>
    <%= f.label :image %><br />
    <%= f.file_field :image %>
  </div>

  <div class="actions">
    <%= f.submit "Create" %>
  </div>

<% end %>

Ok, so the gallery_id is still passed through the URL, but I don’t really see anything wrong with that. You have to pass it somewhere, right? You really only have 3 sane choices on where to pass it: a hidden field, as a querystring parameter, or tucked away inside the URL (nested resource). Of the 3, the latter is IMHO the cleanest method.

If you want to make things even easier on yourself, I highly recommend looking into Jose Valim’s Inherited Resources gem that takes care of a lot of this boilerplate nastiness for you:

https://github.com/josevalim/inherited_resources

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