simple_form undefined method `model_name' for NilClass:Class - ruby-on-rails

Question

I'm learning Rails, and I would like to use simple_form.
I'm creating a simple app (my first one), and I want to create a signup form.
I used rails g scaffold User username:string password:string to create my model and controller.
I'm using Foundation gem too, if that's matter, and I install simple_form with the right command for Foundation.
I've been looking for answers for two hours, I tried many things, I have no idea of what's wrong with my code.
## app/views/home/index.html.erb ##
<%= simple_form_for #User do |f| %>
<%= f.input :username %>
<%= f.input :password %>
<% end %>
## app/models/user.rb ##
class User < ActiveRecord::Base
end
## app/controllers/users_controller.rb
class UsersController < ApplicationController
before_action :set_user, only: [:show, :edit, :update, :destroy]
# GET /users
# GET /users.json
def index
#users = User.all
end
# GET /users/1
# GET /users/1.json
def show
end
# GET /users/new
def new
#user = User.new
end
# GET /users/1/edit
def edit
end
# POST /users
# POST /users.json
def create
#user = User.new(user_params)
respond_to do |format|
if #user.save
format.html { redirect_to #user, notice: 'User was successfully created.' }
format.json { render action: 'show', status: :created, location: #user }
else
format.html { render action: 'new' }
format.json { render json: #user.errors, status: :unprocessable_entity }
end
end
end
# PATCH/PUT /users/1
# PATCH/PUT /users/1.json
def update
respond_to do |format|
if #user.update(user_params)
format.html { redirect_to #user, notice: 'User was successfully updated.' }
format.json { head :no_content }
else
format.html { render action: 'edit' }
format.json { render json: #user.errors, status: :unprocessable_entity }
end
end
end
# DELETE /users/1
# DELETE /users/1.json
def destroy
#user.destroy
respond_to do |format|
format.html { redirect_to users_url }
format.json { head :no_content }
end
end
private
# Use callbacks to share common setup or constraints between actions.
def set_user
#user = User.find(params[:id])
end
# Never trust parameters from the scary internet, only allow the white list through.
def user_params
params.require(:user).permit(:username, :password)
end
end
I'm obviously doing something wrong, and I'm pretty sure I will feel like an idiot, but it's my first app :/
Thanks you.
EDIT: I forgot to say, I tried (desperately) every vars (#user, #users, #User, #Users), any of them doesn't works :/

Answer 1

Get rid of the form from the index.html.erb view (and no, don't put it in show.html.erb either!).
Place this form in the new.html.erb view:
<%= simple_form_for #user do |f| %>
<%= f.input :username %>
<%= f.input :password %>
<%= f.submit %>
<% end %>
Note that the form should reference #user not #User.

Answer 2

Yep ... in the view you are using the #User instance variable (with uppercase U) but in the controller you assign the model to the #user instance variable (with lowecase u) :)

Answer 3

Put #user in lowcase (not #user)
<%= simple_form_for #user do |f| %>
<%= f.input :username %>
<%= f.input :password %>
<% end %>
And save it as new.html.erb view not index.html.erb!