nested_form and strong_parameters issues with 2 nested models forms. The customs gem (https://github.com/inkstak/customs) is used to provide basic resource flow in controllers (hence the `resource_params` method)

form.html.haml

-# Here resource is the timeline

= simple_nested_form_for resource, url: [resource] do |form|
  %table.nested_form
    = form.fields_for :marker_sections, wrapper: false do |section_form|
      %tr.fields
        %td
          %table.nested_form
            %thead
              %tr.section
                %th{colspan: 2}= section_form.input :name
                = nested_form_remove section_form

            %tbody
              = section_form.fields_for :markers, wrapper: false do |marker_form|
                %tr.fields.marker
                  %td= marker_form.input :name
                  %td= marker_form.input :term_at
                  
                  = nested_form_remove marker_form

              = nested_form_add section_form, for: :markers, cols: 3

    = nested_form_add form, for: :marker_sections

  = default_actions

models.rb

class Timeline < ActiveRecord::Base
  has_many :marker_sections
  has_many :markers, through: :marker_sections

  accepts_nested_attributes_for :marker_sections, allow_destroy: true
end

class MarkerSection < ActiveRecord::Base
  belongs_to :timeline
  has_many :markers

  validates :name, presence: true

  accepts_nested_attributes_for :markers, allow_destroy: true
end

class Marker < ActiveRecord::Base
  belongs_to :marker_section

  validates :name, :term_at, :marker_section, presence: true
end

sanitized_timeline_parameters.rb

# It gives a 'Unpermitted parameters: 1389351190703, new_marker_sections' in console
{"marker_sections_attributes"=>{}}

sent_parameters.rb

# This is what the params hash contains prior to sanitization

{
  "utf8"=>"✓", 
  "authenticity_token"=>"*******", 
  "timeline"=> {
    "marker_sections_attributes"=>{
      "1389351190703"=>{
        "name"=>"Foo", 
        "_destroy"=>"false"
      }, 
      "new_marker_sections"=>{
        "markers_attributes"=>{
          "1389351191802"=>{
            "name"=>"Bar", 
            "term_at(3i)"=>"10", 
            "term_at(2i)"=>"1", 
            "term_at(1i)"=>"2014", 
            "_destroy"=>"false"
          }
        }
      }
    }
  }, 
  "id"=>"5"
}

timelines_controller.rb

class TimelinesController < ApplicationController
  load_and_authorize_resource

  def resource_params
    if params[:timeline].present?
      timeline_params = params.require(:timeline).permit(
        :id,
        marker_sections_attributes: [
          :_destroy,
          :id,
          :name,
          :timeline_id,
          markers_attributes: [
            :_destroy,
            :id,
            :name,
            :marker_section_id,
            :term_at
          ]
        ]
      )
      timeline_params
    else
      nil
    end
  end
end

Everything looks good to me, maybe the parameters sent looks a little weird ( for example the marker_sections_attributes is a hash. I guessed it should be an array. And the new_marker_sections looks to be an id ? I did not know this kind of attrs).

Anyway, I've found a pretty good article that may help you on that work, if you did not already found it : http://createdbypete.com/articles/working-with-nested-forms-and-a-many-to-many-association-in-rails-4/

Thanks Louis, the article's example and my situation is quite different, the questions instances already exist when the form is generated, which mean that they already have an id and an index in the survey.questions array.

Here's what I discovered about nested_form (as for version 0.3.2):

1. If I have no sections at all, and if I only create one section and submit the form, the parameters look like:

     {"timeline"=>{"marker_sections_attributes"=>{"1389360179189"=>{"name"=>"Foo", "_destroy"=>"false"}}}}
   

   The record is saved properly.

2. If I have no sections at all, and I create a section and a marker for that section, the parameters looks like:

   {"timeline"=>{
      "marker_sections_attributes"=>{
         "1389360211342"=>{
           "name"=>"Foo", 
           "_destroy"=>"false"
         }, 
         "new_marker_sections"=>{
           "markers_attributes"=>{
             "1389360218312"=>{
               "name"=>"Bar", 
               "term_at(3i)"=>"10", 
               "term_at(2i)"=>"1", 
               "term_at(1i)"=>"2014", 
               "_destroy"=>"false"
             }
           }
         }
       }
     }
   }
   

   Note the new_marker_sections parameter, it seems like it hold attributes of nested models of a to create model. But if I try creating two sections with one marker for each, parameters looks like:

   {
     "timeline"=>{
       "marker_sections_attributes"=>{
         "1389360577960"=>{
           "name"=>"Foo", 
           "_destroy"=>"false"
         }, 
        "new_marker_sections"=>{
          "markers_attributes"=>{
            "1389360580623"=>{
              "marker_section_id"=>"1389360577960", 
              "name"=>"Bar", 
              "term_at(3i)"=>"10", 
              "term_at(2i)"=>"1", 
              "term_at(1i)"=>"2014", 
              "_destroy"=>"false"
            }, 
            "1389360588767"=>{
              "name"=>"Bar 2", 
              "term_at(3i)"=>"10", 
              "term_at(2i)"=>"1", 
              "term_at(1i)"=>"2014", 
              "_destroy"=>"false"
            }
          }
        },
        "1389360582782"=>{
          "name"=>"Foo 2", 
          "_destroy"=>"false"
         }
       }
     }
   }
   

   As you can see here, when we could expect to have one new_marker_sections attribute for each new section there's only one for the first marker_sections_attributes entry.

   In both cases the records aren't saved.

3. If I already have a section (like after step #1), and I add a marker for this section, the parameters looks like:

   {
     "timeline"=>{
       "marker_sections_attributes"=>{
         "0"=>{
           "name"=>"Foo", 
           "_destroy"=>"false", 
           "id"=>"18"
         }, 
        "null"=>{
          "markers_attributes"=>{
            "1389361405773"=>{
              "name"=>"Bar", 
              "term_at(3i)"=>"10", 
              "term_at(2i)"=>"1", 
              "term_at(1i)"=>"2014", 
              "_destroy"=>"false"
            }
          }
        }
      }
   }
   }
   

   Where did come the null key? Of course the marker record isn't saved properly. It goes worse if I had more than one marker or if I also create new section at the same time.

As it is, I must conclude that nested_form is completely unusable. Sad thing.