Passing a local variable into a nested partial - ruby-on-rails

Question

I have a page that renders a collection roughly like this:
index.html.haml
= render partial: 'cars_list', as: :this_car, collection: #cars
_cars_list.html.haml
Edit: _cars_list has other info about the individual car.
%h3 Look at these Cars!
%ul
%li Something about this car
%li car.description
%div
= render partial: 'calendars/calendar_stuff', locals: {car: this_car}
_calendar_stuff.html.haml
- if car.date == #date
%div
= car.date
_cars_contoller.rb
def index
#cars = Car.all
#date = params[:date] ? Date.parse(params[:date]) : Date.today
end
What happens in the calendar stuff partial is that this_car is always the first car in the cars collection, i.e. the same date gets printed over and over.
If I move the logic in _calendar_stuff into the cars_list partial, then the printed result changes as expected.
So it seems that Rails is not passing the local this_car object into the nested partial each time it renders the partial.
Does anyone know why?
P.S. If I structure the code with
#cars.each do |car|
render 'cars_list', locals: {this_car: car}
end
I get the same behavior.

Answer 1

Try this refactoring and see if you get your desired output:
index.html.haml
= render 'cars_list', collection: #cars, date: #date
Get rid of the partial keyword, and pass in the #date instance variable as a local variable to encapsulate the logic in your partials. This point I got from Rails Best Practices.
_cars_list.html.haml
%h3 Look at these Cars!
%ul
%li Something about this car
%div
= render 'calendars/calendar_stuff', car: car, date: date
As you passed in #cars as a collection, this partial will have a reference to a singularized local variable called car, which can then be passed on to the next partial, along with the now-local date variable. Since the partial being rendered is in a different location to here (over under calendars/), the partial keyword is explicitly needed here.
_calendar_stuff.html.haml
- if car.date == date
%div
= car.date
Edit
Suggested to move the call to collection to _cars_list.html.haml, but that wasn't appropriate for the problem.
Edit 2
This is the version of the above code if you still wanted to specify the local variable as this_car, so you would be overriding the car local variable that collection would have generated automatically.
index.html.haml
= render 'cars_list', collection: #cars, as: :this_car, date: #date
_cars_list.html.haml
%h3 Look at these Cars!
%ul
%li Something about this car
%li this_car.description
%div
= render 'calendars/calendar_stuff', this_car: this_car, date: date
_calendar_stuff.html.haml
- if this_car.date == date
%div
= this_car.date