Rails 3-"Couldn't find [model] without an ID" problem - ruby-on-rails

Question

Couldn't find Submission without an ID
This problem has haunted me for last few days, and I can't seem to fix it.
This problem occurs when I try to create an Emailinterest object in the app.
I have two models.
Submission
Emailinterest
For every submission, there can be several emailinterest, but emailinterest cannot exist
C:\Rails\actuirl5\app\controllers\emailinterests_controller.rb
submission_id actually a member of emailinterest object. submission_id is suppose to contain the ID value of submission object.
def create
#emailinterest = Emailinterest.new(params[:emailinterest])
#submission = Submission.find(params[:submission_id])
respond_to do |format|
if #emailinterest.save
Notifier.emailinterest_notification(#emailinterest, #submission).deliver
format.html { redirect_to(#emailinterest, :notice => 'Email was successfully sent!') }
format.xml { render :xml => #emailinterest, :status => :created, :location => #emailinterest }
else
format.html { render :action => "new" }
format.xml { render :xml => #emailinterest.errors, :status => :unprocessable_entity }
end
end
end
C:\Rails\actuirl5\app\views\submissions\show.html.erb
....
<%= render :partial=>"form_new_emailinterest", :locals=>{:emailinterest=>Emailinterest.new(:submission_id=>#submission.id)} %>
C:\Rails\actuirl5\app\views\submissions
<%= form_for(emailinterest) do |f| %>
<%= hidden_field :submission_id, :value => #submission.id %>
<div class="field">
<%= f.label :sender_email %><br />
<%= f.text_field :sender_email %>
</div>
<div class="field">
<%= f.label :sender_email_content %><br />
<%= f.text_area :sender_email_content %>
</div>
<div class="actions">
<%= f.submit %>
</div>
<% end %>
FIX
C:\Rails\actuirl5\app\controllers\emailinterests_controller.rb
def create
#emailinterest = Emailinterest.new(params[:emailinterest])
#submission = Submission.find(params[:submission_id])
respond_to do |format|
if #emailinterest.save
Notifier.emailinterest_notification(#emailinterest, #submission).deliver
format.html { redirect_to(#emailinterest, :notice => 'Email was successfully sent!') }
format.xml { render :xml => #emailinterest, :status => :created, :location => #emailinterest }
else
format.html { render :action => "new" }
format.xml { render :xml => #emailinterest.errors, :status => :unprocessable_entity }
end
end
end
C:\Rails\actuirl5\app\views\submissions_form_new_emailinterest.html.erb
<%= form_for(emailinterest) do |f| %>
<%= hidden_field_tag :submission_id, value = #submission.id %>
<div class="field">
<%= f.label :sender_email %><br />
<%= f.text_field :sender_email %>
</div>
<div class="field">
<%= f.label :sender_email_content %><br />
<%= f.text_area :sender_email_content %>
</div>
<div class="actions">
<%= f.submit %>
</div>
<% end %>
C:\Rails\actuirl5\app\views\submissions\show.html.erb
<%= render :partial=>"form_new_emailinterest", :locals=>{:emailinterest=>Emailinterest.new} %>

Answer 1

Considering you have set #submission in your show action this should work:
<%= hidden_field :submission_id, :value => emailinterest.submission_id %>
and then
#submission = Submission.find(params[:emailinterest][:submission_id])
You cannot access instance variables like #submission in a partials. This is why you have the option :locals.