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

I found the above very useful.

It may help others to share an issue I came across (using Cocoon and Rails with fields_for) if you have more than one level of nesting. That is, if within the _input_fields partial you want another link_to_add_association for a further has_many association.

The problem (here: https://github.com/nathanvda/cocoon/blob/master/lib/cocoon/view_helpers.rb#L95) is that cocoon bases the nested fields for an associated object on a pure instance of the model, rather than that model wrapped in the Reform form object. (Not a criticism, just the way it's designed.) That's fine until you want to render fields for an association on that nested object, because the setup for the nested attributes is defined in your Reform form object, not your model. Essentially f.object in _input_fields is the actual model, not the form object. Unless the model itself has accept_nested_attributes configured, fields_for won't treat the attribute as a nested_attributes_association and the html markup won't name the fields in such a way that reform-rails can parse them into properly nested parameters.

The solution for me was using wrap_object to ensure the record cocoon uses is the model wrapped in the relevant form object, like this:

<%= link_to_add_association 'Add Supporting Image', f, :images, partial: 'web_app/report/supporting_image_fields', force_non_association_create: true, wrap_object: Proc.new {|image| Report::WebAppCreation::ImageWithoutHotspotsForm.new(image) } %>

(There's probably a better way to get the form but this gives the idea).

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