Keep getting "Param is missing or the value is empty" - ruby-on-rails

Question

I keep getting the error mentioned above, even though my form object matches my controller object (most people who had this problem had mixed their objects up in similar questions). What exactly am I doing wrong here?
My code is below:
<!-- The form is here -->
<%= form_for(#user), url: {action: "/users/signup"}, html: {class: "signup_form"} do |f| %>
<%= label_tag(:firstname, "Firstname: ") %>
<%= f.text_field :firstname %>
<%= label_tag(:lastname, "Lastname: ") %>
<%= f.text_field :lastname %>
<%= label_tag(:username, "Username: ") %>
<%= f.text_field :username %>
<%= label_tag(:email, "Email: ") %>
<%= f.text_field :email %>
<%= label_tag(:password, "Password") %>
<%= f.password_field :password %>
<%= f.submit "Create Account" %>
<% end %>
And here's the controller:
class UsersController < ApplicationController
def create
#user = User.new(user_params)
#user.save
redirect_to #user
end
private
def user_params
params.require(:user).permit(:firstname, :lastname, :username,
:email, :password)
end
end
Would really appreciate some help and sorry if I missed something silly here, thanks in advance.

Answer 1

users_controller.rb
class UsersController < ApplicationController
def new
#user = User.new
end
def create
#user = User.new(user_params)
respond_to do |format|
if #user.save
format.html { redirect_to #user, success: 'Thanks for signing up!' }
format.json { render :show, status: :created, location: #user }
else
format.html { render :new }
format.json { render json: #user.errors, status: :unprocessable_entity }
end
end
end
end
users/_form.html.erb
<%= form_for(#user) do |f| %>
<div class="field">
<%= f.label :first_name, "First name" %><br>
<%= f.text_field :first_name %>
</div>
<div class="field">
<%= f.label :last_name, "Last name" %><br>
<%= f.text_field :last_name %>
</div>
<div class="field">
<%= f.label :email, "Email" %><br>
<%= f.text_field :email %>
</div>
<div class="field">
<%= f.label :password, "Password" %><br>
<%= f.password_field :password %>
</div>
<div class="actions">
<%= f.submit "Sign Up" %>
</div>
<% end %>

Answer 2

Why do you have url: {action: "/users/signup"} in the form? It is much simpler if you do the following:
routes.rb
Rails.application.routes.draw do
resources :user
end
views/users/_form.html.erb
<%= form_for(#user), html: {class: "signup_form"} do |f| %>
controllers/users_controller.rb (add the following)
class UsersController < ApplicationController
def new
#user = User.new
end
end
If you do it like this, form_for will automatically detect whether or not the #user object has been saved in the database, and will route the form to the create or update method respectively.
I general I don't reccommend altering the standard RESTful routes unless absolutley necessary. A user is not likely going to look at yoursite.com/users/new in the URL and be upset or confused.