gistfile1.rb

# Models
class Manager::Ad::ThemedCampaign < ActiveRecord::Base
    has_and_belongs_to_many :products
    accepts_nested_attributes_for :products, reject_if: :all_blank, allow_destroy: true
end


class Manager::Ad::Product < ActiveRecord::Base
  has_and_belongs_to_many :themed_campaigns
end


# View
# _form.html.erb
...
<%= f.simple_fields_for :products do |product| %>
    <%= render 'product_fields', f: product %>
<% end %>
...

# _product_field.html.erb
<div class="col-md-2">
    <%= f.input :id %>
</div>