Rails cocoon nested form: undefined method `reflect_on_association' for NilClass:Class

I am trying to build a form with nested resources in my rails 4 app. I am using the cocoon gem. Each step will have substeps, and I'd like to allow the user to add as many substeps to the form and he/she would like.

Step.rb

class Step < ActiveRecord::Base
  has_many :substeps
  accepts_nested_attributes_for :substeps

Substep.rb

class Substep < ActiveRecord::Base
  belongs_to :step

form code

<%= form_for :step, :url => steps_path do |f| %>
  <%= text_field(:step, :title, :value => '', class: 'fly-input input_info', placeholder: 'Process Title', id: 'step_form_title') %>
  <%= text_field(:step, :description, :value => '', class: 'fly-input input_info', placeholder: 'Process Description', id: 'step_form_description') %>
  <%= hidden_field :step, :known %>
  <%= hidden_field_tag :experiment, @experiment.id %>
  <%= f.fields_for :substep do |ff| %>
    <%= ff.text_field :description %>
  <% end %>
  <%= link_to_add_association 'Add substep', f, :substeps %>
  <%= f.submit "Done", class: "main_button" %>
<% end %>

When I do this, I get an error reading: "undefined method `reflect_on_association' for NilClass:Class" on this line

<%= link_to_add_association 'Add substep', f, :substeps %>

Any thoughts on my problem?

EDIT Changed text_field to ff.text_field based on Pavan's suggestion

Solution

Cocoon expects that you provide a form object as second parameter, as you do, but also expects that this second parameter will have actually Rails model instance as attribute f.object.

Your form is created just with model name so form_for :step so Cocoon raises an exception.

To solve this you should change it to form_for @step where @step can be Step.new or any other Step instance.