Rails 4 - has_many through controller actions - ruby-on-rails

Question

I'm trying to set a has_many trough relationship and I'm new to this
#models/partner.rb
class Partner < ActiveRecord::Base
has_many :cards_partners
has_many :cards, through: :cards_partners
#models/card.rb
class Card < ActiveRecord::Base
has_many :cards_partners
has_many :partners, through: :cards_partners
#models/cards_partner.rb
class CardsPartner < ActiveRecord::Base
belongs_to :card
belongs_to :partner
And I specified the controllers
#controllers/cards_controller.rb
before_action :set_card, only: [:show, :edit, :update, :destroy]
def show
#user = #card.user
#partners = #card.partners.order(lastname: :desc)
#partner = #card.partners.new #because I have a modal to create new Partner too
#address is in a polymorphic table. I have to refactor the if-else
if #card.address.street.blank? && #card.address.city.blank?
#address = #user.address
else
#address = #card.address
end
end
def create
#card = current_user.cards.new(card_params)
respond_to do |format|
if #card.save
format.html { redirect_to #card, notice: 'Card was successfully created.' }
format.json { render :show, status: :created, location: #card }
else
format.html { render :new }
format.json { render json: #card.errors, status: :unprocessable_entity }
end
end
end
def update
respond_to do |format|
if #card.update(card_params)
format.html { redirect_to #card, notice: 'Card was successfully updated.' }
format.json { render :show, status: :ok, location: #card }
else
format.html { render :edit }
format.json { render json: #card.errors, status: :unprocessable_entity }
end
end
end
private
def set_card
#card = Card.find(params[:id])
end
def card_params
params.require(:card).permit(partner_ids: [])
end
#controllers/partners_controller.rb
def partner_params
params.require(:partner).permit(card_ids: [])
end
My join table cards_partners is properly set.
To sum up the logic, a User can create Cards and add Partners to this Card.
I set a has_many through between Cards and Partners because I want User to be able to assign already Created Partners to multiple Cards.
I have inside my cards#show view a partial that shows all the partners, and a link to add a new one to this card.
I define my #partners inside my card#show action with (not sure about this since no join specified):
#partners = #card.partners.order(lastname: :desc)
I can't figure how to specify the card and then creating a new partner from card#show view using the link new_partner_path.
Any help would be really appreciated.
Thank you a lot