Reform 1.2.6 nested form (with support for cocoon gem)

graduation_form.rb

# app/forms/graduation_form.rb

require 'concerns/nested_form'

class GraduationForm < Reform::Form
  # Needed for correct behavior of virtual attributes, see https://github.com/apotonick/reform/issues/75
  reform_2_0!

  model :response

  collection :minor_responses, populate_if_empty: MinorResponse do
    include Reform::NestedForm
    property :minor_name
    
    # only save this if minor_name isn't blank
    reject_if_blank :minor_name
  end

end

nested_form.rb

# app/forms/concerns/nested_form.rb

# Helps facilitates nested forms. loosely based on: https://github.com/apotonick/reform/issues/86#issuecomment-73918238
# Adds the ability to exclude saving if a particular field is blank, and helper methods to make cocoon gem work.

module Reform
  module NestedForm
    extend ActiveSupport::Concern

    included do
      property :id, virtual: true
      property :_destroy, virtual: true
      @reject_field = []
    end

    def sync_hash(options)
      if fields._destroy == '1' || reject_fields?
        model.mark_for_destruction
      end
      super(options)
    end

    def new_record?
      model.new_record?
    end

    def marked_for_destruction?
      model.marked_for_destruction?
    end

    def reject_fields?
      self.class.reject_field.any? { |f| fields[f].blank? }
    end

    class_methods do
      def reject_if_blank(field)
        @reject_field << field
      end

      def reject_field
        @reject_field
      end
    end

  end
end

response.rb

# app/models/response.rb

class Response < ActiveRecord::Base
  # autosave: true is needed to get mark_for_destruction
  has_many :minor_responses, dependent: :destroy, autosave: true
end

Here is an update that works with Reform 2.2.1:

https://gist.github.com/lucaspiller/615f09bb525a14163921fd56b4b8e611