I have a simple has_many belongs_to relationship and I want to include the parent object for the view of the belongs_to model and I have had some success, but I want it to work better.

class Submission < ActiveRecord::Base
  belongs_to :user
  belongs_to :contest
end
class Contest < ActiveRecord::Base
  belongs_to :user
  has_many :submissions, :dependent => :destroy
end

In the case that works, I pass contest_id to submissions by placing it in the url:

   <%= link_to 'Submit Contest Entry', new_submission_path(:contest_id => @contest.id), 
                            :class => 'btn btn-primary btn-large mleft10' %>

So that, combined with a hidden_field:

 <%= f.hidden_field :contest_id %>

And a find_contest method in the controller (called with a before_filter):

def find_contest
  #the next line is giving the error (line 76)
  @contest = Contest.find(params[:submission][:contest_id])
end

Makes it work for submissions/new, but how do I add a find to the controller that just works across more than that one page, like if I want to access in show and index. Right now, I get an error:

Started GET "/submissions?contest_id=5" for 127.0.0.1 at 2012-12-01 16:01:45 -0800
Processing by SubmissionsController#index as HTML
  Parameters: {"contest_id"=>"5"}
  User Load (0.3ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 2 ORDER BY users.created_at DESC LIMIT 1
Completed 500 Internal Server Error in 37ms

NoMethodError (undefined method `[]' for nil:NilClass):
  app/controllers/submissions_controller.rb:76:in `find_contest'

[edited] Adding show action for submissions:

before_filter :find_contest, :except => [:index, :show, :edit, :update, :destroy]

def find_contest
  @contest = Contest.find(params[:submission][:contest_id])
end

def show
  contest_id = @submission.contest_id
  @submission = @commentable = Submission.find(params[:id])
  @comments = @commentable.comments.order(:created_at).reverse
  respond_to do |format|
    format.html  # show.html.erb
    format.json  { render :json => @submission }
  end
end