post model

class Post < ActiveRecord::Base
  belongs_to :user
  default_scope -> { order('created_at DESC') }
  validates :content, presence: true, length: {maximum: 140}
  validates :user_id, presence: true
end

sessions helper

module SessionsHelper
  def sign_in(user)
    remember_token = User.new_remember_token
    cookies.permanent[:remember_token] = remember_token
    user.update_attribute(:remember_token, User.encrypt(remember_token))
    self.current_user = user
  end
  
  def signed_in?
    !current_user.nil?
  end
  
  def sign_out
    self.current_user = nil
    cookies.delete(:remember_token)
  end
 
  def current_user=(user)
    @current_user = user
  end

  def current_user
    remember_token = User.encrypt(cookies[:remember_token])
    @current_user ||= User.find_by(remember_token: remember_token)
  end
  
  def current_user?(user)
    user == current_user
  end
  
  def redirect_back_or(default)
    redirect_to(session[:return_to] || default)
    session.delete(:return_to)
  end

  def store_location
    session[:return_to] = request.url if request.get?
  end
  def signed_in?
                !current_user.nil?
  end
end

user model

class User < ActiveRecord::Base
  has_many :posts, dependent: :destroy
  before_save { self.email = email.downcase}
  before_create :create_remember_token
  has_secure_password
  
  before_validation :downcase_email
  validates :name, presence: true, length: {maximum: 50}
  VALID_EMAIL_REGEX = /\A[\w+\-.]+@[a-z\d\-.]+\.[a-z]+\z/i
  validates :email, presence: true, format: { with: VALID_EMAIL_REGEX}, uniqueness: {case_sensitive: false}
  validates :password, length: {minimum: 3}
  
  def User.new_remember_token
    SecureRandom.urlsafe_base64
  end
  def User.encrypt(token)
    Digest::SHA1.hexdigest(token.to_s)
  end
private
  def downcase_email
    self.remember_token = User.encrypt(User.new_remember_token)
    self.email = self.email.downcase if self.email.present?
  end
  def create_remember_token
      self.remember_token = User.encrypt(User.new_remember_token)
    end
end