using 'reform' gem with nested routing - ruby-on-rails

Question

i want to use 'reform' gem to create object with nested attribute. i have models:
class Dish < ActiveRecord::Base
belongs_to :lunch_set
end
class Side < ActiveRecord::Base
belongs_to :lunch_set
end
class LunchSet < ActiveRecord::Base
belongs_to :restaurant
belongs_to :day
has_one :dish
has_many :sides
end
lunchset controller 'new' method:
def new
#lunch_set = #restaurant.lunch_sets.build
#form = LunchSetForm.new(dish: Dish.new, side: Side.new)
respond_to do |format|
format.html # new.html.erb
format.json { render json: #lunch_set }
end
end
routes file:
namespace :admin do
resources :restaurants do
resources :lunch_sets
resources :days do
resources :lunch_sets
end
end
end
and LunchSetForm
class LunchSetForm < Reform:Form
include DSL
include Reform::Form::ActiveRecord
property :name, on: :dish
property :name, on: :side
end
my question is how to construct views/admin/lunch_sets/_form.html , especially considering those routes? when i tried
= simple_form_for #form do |f|
= f.input :name
= f.input :name
.actions
= f.submit "Save"
but it gives me error
undefined method `first' for nil:NilClass
and points into line
= simple_form_for #form do |f|

Answer 1

form_for (and in turn, simple_form_for) expect the form object to have ActiveModel methods like model_name to figure out how to name your form and its inputs and to resolve the form's action url. You're close to getting it right by including Reform::Form::ActiveRecord, but there are a couple more things you need to do:
require 'reform/rails'
class LunchSetForm < Reform:Form
include DSL
include Reform::Form::ActiveRecord
property :name, on: :dish
property :name, on: :side
model :dish
end
The model :dish line tells Reform that you want the form's 'main model' to be the Dish instance. This means that it will make your form respond to the methods that ActiveModel usually provides for normal Rails models using the 'main model' to provide the values for those methods. Your form input names will look like dish[name] etc and it will post to your dishes_url. You can set your model to anything you like, but an instance whatever you choose needs to be passed into the form constructor.