Param is missing or value is empty until I refresh page - ruby-on-rails

Question

Hi guys I have really strange problem.I have events and enrollments. Every time when I want to make an enrollment for specific event Rails show me:
Error: Param is missing or value is empty: enrollment
When I refresh the page everything is Ok.
new view code:
<%=form_for :enrollment do |f|%>
<%= f.select(:car_id) do %>
<% current_user.cars.each do |c| %>
<%= content_tag(:option, c.mark, value: c.id) %>
<% end %>
<% end %>
<%=f.submit :save %>
<%end%>
Strong params:
def enrollment_params
params.require(:enrollment).permit(:car_id)
end
create method:
def create
#event=Event.find(params[:id])
#enrollment = current_user.enrollments.build(enrollment_params)
#enrollment.save
#event.enrollments<<#enrollment
end
new action:
def new
#enrollment=Enrollment.new
end
Log:
Started POST "/events/39/enrollments/new" for 127.0.0.1 at 2016-05-21 22:53:52 +0300
Processing by EnrollmentsController#create as HTML
Parameters: {"authenticity_token"=>"b4oZl8n8ALq1fgmqweygyWDF8oc7Ew3nelvhst5SRJkza8x92VNo3fXg+/gqw+NfKVPYvJfu9LCthRBGFB0xSA==", "id"=>"39"}
Event Load (0.1ms) SELECT "events".* FROM "events" WHERE "events"."id" = ? LIMIT 1 [["id", 39]]
User Load (0.1ms) SELECT "users".* FROM "users" WHERE "users"."id" = ? ORDER BY "users"."id" ASC LIMIT 1 [["id", 12]]
Completed 400 Bad Request in 5ms (ActiveRecord: 0.2ms)
Routes:
Prefix Verb URI Pattern Controller#Action
new_user_session GET /users/sign_in(.:format) devise/sessions#new
user_session POST /users/sign_in(.:format) devise/sessions#create
destroy_user_session DELETE /users/sign_out(.:format) devise/sessions#destroy
user_password POST /users/password(.:format) devise/passwords#create
new_user_password GET /users/password/new(.:format) devise/passwords#new
edit_user_password GET /users/password/edit(.:format) devise/passwords#edit
PATCH /users/password(.:format) devise/passwords#update
PUT /users/password(.:format) devise/passwords#update
cancel_user_registration GET /users/cancel(.:format) devise/registrations#cancel
user_registration POST /users(.:format) devise/registrations#create
new_user_registration GET /users/sign_up(.:format) devise/registrations#new
edit_user_registration GET /users/edit(.:format) devise/registrations#edit
PATCH /users(.:format) devise/registrations#update
PUT /users(.:format) devise/registrations#update
DELETE /users(.:format) devise/registrations#destroy
user_cars GET /users/:user_id/cars(.:format) cars#index
POST /users/:user_id/cars(.:format) cars#create
new_user_car GET /users/:user_id/cars/new(.:format) cars#new
edit_user_car GET /users/:user_id/cars/:id/edit(.:format) cars#edit
user_car GET /users/:user_id/cars/:id(.:format) cars#show
PATCH /users/:user_id/cars/:id(.:format) cars#update
PUT /users/:user_id/cars/:id(.:format) cars#update
DELETE /users/:user_id/cars/:id(.:format) cars#destroy
users GET /users(.:format) users#index
POST /users(.:format) users#create
new_user GET /users/new(.:format) users#new
edit_user GET /users/:id/edit(.:format) users#edit
user GET /users/:id(.:format) users#show
PATCH /users/:id(.:format) users#update
PUT /users/:id(.:format) users#update
DELETE /users/:id(.:format) users#destroy
event_comments GET /events/:event_id/comments(.:format) comments#index
POST /events/:event_id/comments(.:format) comments#create
new_event_comment GET /events/:event_id/comments/new(.:format) comments#new
edit_event_comment GET /events/:event_id/comments/:id/edit(.:format) comments#edit
event_comment GET /events/:event_id/comments/:id(.:format) comments#show
PATCH /events/:event_id/comments/:id(.:format) comments#update
PUT /events/:event_id/comments/:id(.:format) comments#update
DELETE /events/:event_id/comments/:id(.:format) comments#destroy
event_results GET /events/:event_id/results(.:format) results#index
POST /events/:event_id/results(.:format) results#create
new_event_result GET /events/:event_id/results/new(.:format) results#new
edit_event_result GET /events/:event_id/results/:id/edit(.:format) results#edit
event_result GET /events/:event_id/results/:id(.:format) results#show
PATCH /events/:event_id/results/:id(.:format) results#update
PUT /events/:event_id/results/:id(.:format) results#update
DELETE /events/:event_id/results/:id(.:format) results#destroy
event_enrollments GET /events/:event_id/enrollments(.:format) enrollments#index
POST /events/:event_id/enrollments(.:format) enrollments#create
new_event_enrollment GET /events/:event_id/enrollments/new(.:format) enrollments#new
edit_event_enrollment GET /events/:event_id/enrollments/:id/edit(.:format) enrollments#edit
event_enrollment GET /events/:event_id/enrollments/:id(.:format) enrollments#show
PATCH /events/:event_id/enrollments/:id(.:format) enrollments#update
PUT /events/:event_id/enrollments/:id(.:format) enrollments#update
DELETE /events/:event_id/enrollments/:id(.:format) enrollments#destroy
events GET /events(.:format) events#index
POST /events(.:format) events#create
new_event GET /events/new(.:format) events#new
edit_event GET /events/:id/edit(.:format) events#edit
event GET /events/:id(.:format) events#show
PATCH /events/:id(.:format) events#update
PUT /events/:id(.:format) events#update
DELETE /events/:id(.:format) events#destroy
GET /attend/:id(.:format) events#attend
POST /cancel/:id(.:format) events#cancel
all GET /all(.:format) users#all
POST /all(.:format) users#all
myevents GET /myevents(.:format) users#myevents
POST /myevents(.:format) users#myevents
POST /events/:id/comments/new(.:format) comments#create
GET /events/:id/comments(.:format) comments#index
PUT /events/:id/comments/:id/edit(.:format) comments#edit
POST /events/:id/like(.:format) events#like
GET /events/:id/like(.:format) events#like
POST /events/:id/unlike(.:format) events#unlike
GET /events/:id/unlike(.:format) events#unlike
POST /users/:id/cars/new(.:format) cars#create
DELETE /events/:id/enrollments/:id(.:format) enrollments#destroy
GET /users/:id/usercars(.:format) users#usercars
GET /users/:id/userevents(.:format) users#userevents
POST /events/:id/results/new(.:format) results#create
GET /events/:id/statistics(.:format) results#statistics
GET /events/:id/classfwd(.:format) results#classfwd
GET /events/:id/classrwd(.:format) results#classrwd
GET /events/:id/classawd(.:format) results#classawd
GET /events/:id/enrollments(.:format) enrollments#index
GET /users/:id/mygallery(.:format) users#mygallery
root GET / welcome#Welcome

Answer 1

Your error log shows you are trying to commit a post when going to "/events/39/enrollments/new". This route needs to be a 'get' request or it will try to post to the server without any parameters included! Go into your config/routes file and change that from a 'post' to a 'get' route and this shouldn't happen.
Edit: You need to include the nested attribute in your form:
<%= form_for([#event, #enrollment]) do |f| %>
You then need to define #event in your new action with the params being passed in the url to it as you do in the create action. Using these objects instead of :enrollment should also guide it to using the right action (create) instead of posting to new.