rails nested_forms error and logic - ruby-on-rails

Question

I'am trying to add some fields to my nested form. I've included the gem nested_forms (https://github.com/ryanb/nested_form).
For my prebuilt maps, it works fine, but i can't add new fields.
My controller:
def new
#people = Person.all
#vehicles = Vehicle.all
#roles = Role.all
#pratice_people = []
#people.each do |a|
if a.at1 == true
#pratice_people << a
end
end
#practice = Practice.new
#pratice_people.count.times { #practice.uebung_maps.build }
render action: "new"
end
and my form:
<% #runs = 0 %>
<%= f.fields_for :uebung_maps do |map| %>
<tr>
<%= map.hidden_field :role_id, :id => "role_id_#{#runs}" %>
<%= map.hidden_field :vehicle_id, :id => "vehicle_id_#{#runs}" %>
<%= map.hidden_field :person_id , :value => #pratice_people[#runs].id %><br/>
<td><%= #pratice_people[#runs].name %></td>
<td><%= map.select :role_id, options_from_collection_for_select(#roles, :id, :name), :include_blank => true %></td>
<td><%= map.select :vehicle_id, options_from_collection_for_select(#vehicles, :id, :name), :include_blank => true %></td>
<td><%= map.text_field :time %></td>
</tr>
<% #runs += 1 %>
<% end %>
<%= f.link_to_add "+" , :uebung_maps %>
If i try to access the page, i get following error report
Called id for nil, which would mistakenly be 4 -- if you really wanted the id of nil, use object_id
Do I have to (or how to) create a logic to rerun Practice.uebung_maps.build?, because I thought this is done within the nested_forms gem....

Answer 1

First, make sure the models are created correctly.
class Practice < ActiveRecord::Base
has_many :uebung_maps
accepts_nested_attributes_for :uebung_maps
end
class UebungMap < ActiveRecord::Base
end
Second, make sure the form_for is nested correctly
<%= nested_form_for #practice do |f| %>
<%= f.fields_for :uebung_maps do |uebung_maps_form| %>
<%= uebung_maps_form.text_field :time %>
<% end %>
<p><%= f.link_to_add "+", :uebung_maps %></p>
<% end %>