Setting an object attribute from a hidden field Ruby on Rails -

i'm trying update idea attribute challenge_id through hidden form field. here field:

<%= f.hidden_field :challenge_id, :value => @challenge.id %>

it passes challenge id param when idea created idea controller#create method:

started post "/ideas.js" ::1 @ 2015-06-18 15:39:49 -0400 processing ideascontroller#create js parameters: {"utf8"=>"✓", "idea"=>{"title"=>"adsf", "description"=>"asf", "domain_tokens"=>"20", "challenge_id"=>"5"}, "commit"=>"create idea"}

this challenge_id => 5 should saved idea in line @idea = idea.new(idea_params) below:

ideas_controller.rb

def create     @idea = idea.new(idea_params)      respond_to |format|       if @idea.save         idea_count = @idea.user.ideas_created_count         @idea.user.update(:ideas_created_count => idea_count + 1)         @idea.domains.each |domain|           current_user.add_points(1, category: domain.title)         end         @ideas = current_user.current_team.ideas.sort_by{|i| i.heat_index}.reverse         @ideas = @ideas.paginate(:page => params[:ideas_page], :per_page => 10)         format.html { redirect_to :back, notice: 'idea created.' }         format.json { render :show, status: :created, location: @idea }         format.js       else         format.html { redirect_to :back, notice: "you must attach domains idea." }         format.json { render json: @idea.errors, status: :unprocessable_entity }         format.js { render :create_failed }       end     end   end   .   .   def idea_params    params.require(:idea).permit(:title, :description, :challenge_id)   end 

however, challenge id isn't being saved idea other permitted idea_params, :title , :description. how can challenge_id saved idea when it's created?

instead of using hidden field, why not pass in challenge_id in form? otherwise, leave open possibility users can enter whatever want in hidden field.

form:

form_for [@challenge, idea.new] |f| 

and then:

def create     @challenge = challenge.find(params[:challenge_id])     @idea = idea.new(idea_params)     @idea.challenge_id = @challenge.id end