Rails - Simple Forms - undefined method `model_name' for nil:NilClass

Question

I am trying to make a form where you select tables that you want to export. I made a simple form with a list of tables that can be exported. My plan was to allow the user to toggle check boxes for the tables they want to export and as a result they would be able to download a zip file containing the tables.

Currently, when I try to go to the page with the form, I get an error:

undefined method 'model_name' for nil:NilClass

The majority of the usage of simple forms that I see online consists of using forms to create new items to save in their models. As a result, it seems that the line simple_form_for @example would mean that when the user clicks the submit button, there is a line in the controller such as @example = SomeClass.new". My understanding is that the user input of the form is saved in @example and can be used by the controller. However, as I am not creating a new item in the model, I just want to use the values from @example, I am not sure what to put in the controller to get rid of the error so that I can code the rest of the function in the controller.

Controller:

class FormController < ApplicationController

  def index
    @options = []
    print(@options)
  end

end

The form used:

<h2>Which tables do you want to export?</h2>
<div class="well">
  <% tables_in_model = %w(Table1 Table2 Table3) %>
  <%= simple_form_for @selected_options, :url => form_index_path, :method => :get do |f| %>
    <%= f.input :options, as: :check_boxes, collection: tables_in_model %>
    <%= f.button :submit, as: :Submit %>
  <% end %>
</div>

Answer 1

As you said correctly in your question, simple_form should be used to render forms to the user when her actions are related to the creation or edition of ActiveRecord models.

For instance, when writing down code to enable a search feature, where your goal is to simply pass a bunch of user chosen params to a controller, you should not use it. I believe you are in a similar position with the feature you described.

Simple solution though: use rails form related DSL to get your form going!

Hope it's the answer you needed. Feel free to ask for more details if needed. Cheers!