ActiveModel::ForbiddenAttributesError when creating new user

I have this model in Ruby but it throws a ActiveModel::ForbiddenAttributesError

class User < ActiveRecord::Base
  attr_accessor :password
  validates :username, :presence => true, :uniqueness => true, :length => {:in => 3..20}
  VALID_EMAIL_REGEX = /\A[\w+\-.]+@[a-z\d\-.]+\.[a-z]+\z/i
  validates :email, presence: true, :uniqueness => true, format: { with: VALID_EMAIL_REGEX }

  validates :password, :confirmation => true
  validates_length_of :password, :in => 6..20, :on => :create

  before_save :encrypt_password
  after_save :clear_password

  def encrypt_password
    if password.present?
      self.salt = BCrypt::Engine.generate_salt
      self.encrypted_password= BCrypt::Engine.hash_secret(password, salt)
    end
  end

  def clear_password
    self.password = nil
  end
end

when I run this action

  def create
    @user = User.new(params[:user])
    if @user.save
      flash[:notice] = "You Signed up successfully"
      flash[:color]= "valid"
    else
      flash[:notice] = "Form is invalid"
      flash[:color]= "invalid"
    end
    render "new"
  end

on ruby 1.9.3p194 (2012-04-20 revision 35410) [x86_64-linux].

Can you please tell me how to get rid of this error or establish a proper user registration form?

I guess you are using Rails 4. If so, the needed parameters must be marked as required.

You might want to do it like this:

class UsersController < ApplicationController

  def create
    @user = User.new(user_params)
    # ...
  end

  private

  def user_params
    params.require(:user).permit(:username, :email, :password, :salt, :encrypted_password)
  end
end

For those using CanCan. People might be experiencing this if they use CanCan with Rails 4+. Try AntonTrapps's rather clean workaround solution here until CanCan gets updated:

In the ApplicationController:

before_filter do
  resource = controller_name.singularize.to_sym
  method = "#{resource}_params"
  params[resource] &&= send(method) if respond_to?(method, true)
end

and in the resource controller (for example NoteController):

private
def note_params
  params.require(:note).permit(:what, :ever)
end

Update:

Here's a continuation project for CanCan called CanCanCan, which looks promising:

CanCanCan