Reform 2.2 nested form (with support for cocoon gem)

_fields.html.slim

= f.simple_fields_for :inputs do |input|
  = render 'input_fields', f: input
.links
  = link_to_add_association f, :inputs, partial: 'input_fields', force_non_association_create: true do
    Add

_input_fields.html.slim

.nested-fields
  = f.input :foo, as: :string
  = f.input :bar, as: :string

  = f.hidden_field :_destroy
  = f.hidden_field :id

  = link_to_remove_association f do
    Delete

form.rb

class IndicatorForm < Reform::Form
  InputPrepopulator = -> (options) {
    if inputs.size == 0
      inputs << IndicatorInput.new
    end
  }

  InputPopulator = -> (options) {
    fragment, collection, index = options[:fragment], options[:model], options[:index]

    if fragment["_destroy"] == "1"
      # Marked for removal
      collection.delete_at(index)
      return skip!
    else
      (item = collection[index]) ? item : collection.insert(index, IndicatorInput.new).
    end
  }

  collection :inputs, inherit: true, prepopulator: InputPrepopulator, populator: InputPopulator do
    include NestedForm
  
    property :foo
    property :bar
  end
end

nested_form.rb

module NestedForm
  extend ActiveSupport::Concern

  included do
    property :id,       writeable: false
    property :_destroy, virtual: true
  end

  def new_record?
    model.new_record?
  end

  def marked_for_destruction?
    model.marked_for_destruction?
  end
end

This is really helpful, link_to_add_association raises undefined method reflect_on_association' for NilClass:Class`

in order to solve it

class IndicatorForm < Reform::Form
  ...
  def self.reflect_on_association(obj)
      Indicator.reflect_on_association(obj)
  end
end