Rails - User Pressing 'Back' after object creation, Creating Duplicates - ruby-on-rails

Question

I'm having a problem where when a user fills out my evaluation form, click "Create", then click the browser's back button, make some edits, and click "Create" again, it's creating duplicate Evaluations.
What is the best way to prevent something like this happening.
Only ONE evaluation should exist for each survey_criterion on creation. I don't want the user to lose any data they enter after hitting the back button, filling out the form with new stuff, and clicking "Create" again.
UPDATE
routes.rb
resources :survey_criteria do
resources :groups do
resources :evaluations
end
end
survey_criterion.rb
has_many :evaluations
evaluation.rb
belongs_to :survey_criterion
belongs_to :group
There are more complicated associations, but the answer I'm looking for is more, "how does one handle it when users press the 'Back' button, modify the form, then click Create again".
I want it to update the one that was automatically created I think in this instance, and not throw an error to the user. I know I could add a validation that would error out, but I want this to be invisible to the user I think.
Thoughts?

Answer 1

The simplest solution, would be to change the create action, which should work like this pseudocode:
def create
# ...
if evaluation_exists?
update_evaluation(params[:evaluation])
else
create_evaluation(params[:evaluation])
end
# ...
end
As for Your question "how does one handle it when users press the 'Back' button, modify the form, then click Create again", then I use some random token (a short string) placed as a hidden field in the form.
When the create-request comes, I check whether this token is already stored in the session. If it is not, then I create the object, and add that token to the list of used ones. If the token is already present in the session, I know that user has just resubmitted the form, and I can act accordingly. Usually I ask him whether another object should be created. In the session I store usually not more that 3-5 tokens.
It looks like this (yes, that's just an illustration):
def create
token = params[:token]
session[:tokens] ||= []
if session[:tokens].include? token
render_the_form_again( "You have already created the object. Want another?" )
else
create_the_object
session[:tokens] << token
end
# ...
end

Answer 2

In your Evaluation model, add this line :
validates_uniqueness_of :survey_criterion_id
This is assuming that SurveyCriterion holds the foreign key that associates with your Evaluation.

Answer 3

You can also do 2 things :
Prevent the browser cache.
Disable the Create button with :disable_with => "Processing" option.
It is discussed here too: https://stackoverflow.com/a/12112007/553371

Answer 4

A less elegant way but more generic way to do this is to use history.pushState. On the page after create:
$(function(){
if(history.pushState){
window.onpopstate = function(event){
if(window.history.state && window.history.state.previousStep){
window.location = window.history.state.previousStep;
}
}
window.history.replaceState({ previousStep: '#{edit_resource_url(resource)}'}, document.title, window.location);
window.history.pushState({}, document.title, window.location);
}
})
This example uses HTML5's History API. A similar thing can be done with fallback using the history.js project