How to connect two exisitng Objects in Rails via onclick?

I have User Model and Car Model.

I show a add button in the cars/index.html.erb for every Car so that a User can add any Car he want. And in the users/show.html.erb i want to list all the Cars he added.

The Problem is, it connect not only the clicked Car to the User, but also all other cars in the cars/index.html.erb.

user.erb and car.erb

class User << ActiveRecord::Base
   has_many :cars 
end

class Car << ActiveRecord::Base
   belongs_to :user
end

cars/index.html.erb

 <% @car.each do |car| %>
    #...
    <td><%= link_to 'Get Car', '#', :onclick => get_car(car.id) %></td>
    #...
 <% end %>

cars_helper.rb

def get_car(id)
 current_user.cars << Car.find_by(id: id)
end

user/show.html.erb

<% if @user.cars.any? %>  
<% @user.cars.each do |c| %>
   <%= c.name %>
<% end %>