Form for has_many :through - ruby-on-rails

Question

Could you please show how form should look for has_many :through association?
user.rb
has_many :participations
has_many :events, :through => :participations
event.rb
has_many :participations
has_many :users, :through => :participations
accepts_nested_attributes_for :participations
participations.rb
belongs_to :user
belongs_to :event
events_controller.rb
def new
#event = Event.new
#participation = #event.participations.build
end
def create
#event = Event.new(params[:event].permit!)
respond_to do |format|
if #event.save
format.html { redirect_to events_path, notice: 'Event was successfully created.' }
else
format.html { render action: 'new' }
end
end
end
But I have some doubts about
_form.html.erb
<%= form_for #event, :html => {:class => 'row'} do |f| %>
<div class="form-group">
<%= f.label :name, :class => 'control-label' %><br>
<%= f.text_field :name, :class => 'form-control' %>
</div>
<div class="form-group">
<%= f.label :description, :class => 'control-label' %><br>
<%= f.text_area :description, :class => 'form-control' %>
</div>
<div class="form-group">
<%= f.fields_for :participations do |f_p| %>
<%= f_p.label :user_ids, "Users" %>
<%= f_p.select :user_ids, options_from_collection_for_select(User.all, :id, :name), { :multiple => true } %>
<% end %>
</div>
<div class="actions">
<%= f.submit 'Save', :class => 'btn btn-default' %>
</div>
<% end %>
What is the best way to add Users for a new Event?
Thanks in advance!

Answer 1

Instead of
<div class="form-group">
<%= f.fields_for :participations do |f_p| %>
<%= f_p.label :user_ids, "Users" %>
<%= f_p.select :user_ids, options_from_collection_for_select(User.all, :id, :name), { :multiple => true } %>
<% end %>
</div>
You could try:
<div class="form-group">
<% User.all.each do |user| %>
<%= check_box_tag 'event[user_ids]', user.id, #event.user_ids.include?(user.id) -%>
<%= label_tag user.name %>
<% end %>
</div>

Answer 2

Just found more elegant solution:
form.html.erb
<div class="form-group">
<%= f.label :user_ids, "Participants" %><br>
<%= f.collection_select :user_ids, User.order(:name), :id, :name, {}, {:multiple => true} %>
</div>
more you can find here: http://railscasts.com/episodes/258-token-fields-revised