Missing required key :id on new photo form - ruby-on-rails

Question

I have Photo model which belongs to User model. Rails returns this error whenever I try to access new view for /users/4/photos/new:
No route matches {:action=>"index", :controller=>"photos", :user_id=>"4"} missing required keys: [:id]
This is my PhotosController:
class PhotosController < ApplicationController
def index
find_user
#photos = #user.photos
end
def new
find_user
#photo = #user.photos.build
end
def show
find_photo
end
def create
#photo = Photo.new(params[:photo])
if #photo.save
if params[:images]
params[:images].each { |image|
#user.photos.create(image: image)
}
end
else
render 'new'
end
end
def update
find_user
#photo = #user.photos.find(params[:id])
if #photo.update
redirect_to photos_path
else
render 'edit'
end
end
def destroy
find_photo
#photo.destroy
redirect_to photos_path
end
private
def find_user
#user = User.find(params[:user_id])
end
def find_photo
#photo = Photo.find(params[:id])
end
def photo_params
require(:photo).permit(:title, :image, :user_id)
end
end
And this is my _form partial for photos views.
<%= form_for #photo, :html => { class: 'form-horizontal', multipart: true } do |f| %>
<%= f.label :title %>
<%= f.text_field :title, class: "input-field form-control" %>
<%= f.label :pictures %>
<%= file_field_tag "images[]", type: :file, multiplue: true %>
<%= f.submit nil, class: "btn btn-primary" %>
I don't understand how ID is missing, I'm providing it via find_user method.
Update:
# routes.rb
Rails.application.routes.draw do
root 'welcome#index'
# prevedene rute
get "/pocetna" => "welcome#index", as: "index"
get "/o-sajtu" => "welcome#about", as: "about"
get "/moj-profil" => "users#show", as: "profile"
post "/users" => "users#create"
get "/users" => "users#index", as: "users"
get "/user" => "users#index"
get "/users/:id" => "users#show", as: "user"
get "/users/:id/edit" => "users#edit", as: "edit_user"
patch "/users/:id" => "users#update"
get "/users/add" => "users#new", as: "new_user"
resources :sessions, only: [:new, :create, :destroy]
get "/login" => "sessions#new", as: "login"
post "/login" => "sessions#create"
get "/logout" => "sessions#destroy", as: "logout"
delete "/logout" => "sessions#destroy"
resources :users do
resources :photos
end
end
Update
I get new error now
undefined method `photos_path' for #<<Class:0x007f4f14f16160>:0x00000004f040e8>
First line of the form is marked.

Answer 1

change your form_for in _form partial to
form_for([#user, #photo]) do |f|
#rest of your code
end

Answer 2

How do you call the /users/4/photos/new in your view? Ideally you should pass the user_id as id not as user_id. Once, you do that this issue should be resolved.