No route matches [POST] in Ruby app I'm building - ruby-on-rails

Question

I'm trying to build an application that lets users create a trip with restful routing however, I'm encountering an issue when a user is logged in and they try to create a trip.
I get "No route matches [POST] "/users/8/trips/new""
These are the routes:
resources :users do
resources :trips
end
This is the trip controller:
class TripsController < ApplicationController
def new
#trip = Trip.new
end
def create
#trip = Trip.create(trip_params)
redirect_to root_path
end
end
This is the form to create a new trip. This is where I click submit and get the error:
<div class="trip_form">
<%= form_for :trip do |f| %>
<%= f.label :where, "Where?" %><br/>
<%= f.text_field :where, placeholder: "Hawaii" %><br>
<%= f.label :when, "When?" %><br/>
<%= f.text_field :when, placeholder: "#" %><br>
<%= f.label :price_per_person, "Price per person? (Approximately)" %><br/>
<%= f.text_field :price_per_person, placeholder: "$550" %><br>
<%= f.submit "Create Trip Idea"%>
<% end %>
These are the routes:
$ rake routes
Prefix Verb URI Pattern Controller#Action
user_trips GET /users/:user_id/trips(.:format) trips#index
POST /users/:user_id/trips(.:format) trips#create
new_user_trip GET /users/:user_id/trips/new(.:format) trips#new
edit_user_trip GET /users/:user_id/trips/:id/edit(.:format) trips#edit
user_trip GET /users/:user_id/trips/:id(.:format) trips#show
PATCH /users/:user_id/trips/:id(.:format) trips#update
PUT /users/:user_id/trips/:id(.:format) trips#update
DELETE /users/:user_id/trips/:id(.:format) trips#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
root GET / users#index
I thought I should be able to just fill out the trip new form and when I press submit for it to
automatically communicate with the create method in the trip controller? When I do change :trip to #trip in the form I get this error:
NoMethodError in Trips#new
undefined method `trips_path'
Thanks!

Answer 1

<%= form_for :trip do |f| %>
should be
<%= form_for [#user, #trip] do |f| %>
and because you're using nested resources you'll need this too
# controller
def new
#user = User.find(1)
#trip = Trip.new
end
More in the documentation for form_for

Answer 2

There is a tiny mistake in your form_for helper. It should be:
<%= form_for #trip do |f| %>
...
The helper will look at the #trip object and see that it is a new object (= not yet saved to the database) and thus will choose POST /users/8/trips as the form action.
By not handing an ActiveRecord Object to the helper, the generated HTML form has no action, so submitting the form will POST to the current path (which is the new-path)
EDIT
In order to let the helper choose a route for the nested resource situation you have, use:
<%= form_for [current_user, #trip] do |f| %>
...
assuming that current_user is the user object you are referring to.