NoMethodError: undefined method `has_many' for ActiveRecord:Module

I am trying to add user to table User by console : u=User.new but I always get an error undefined method `has_many' for ActiveRecord:Module and I tried many solutions but nothing helped so I tried to remove has_many relations ot avoid the error I get the same error but with undefined method 'attr_accessor' or I get when I remove attr_accessor undefined method 'before_save' . i have another 4 models which are message-post-friendship-comment.

here is my User Model

 class User < ActiveRecord::


  has_many :posts
  has_many :comments
  has_many :messages
  has_many :friendships
  has_many :friends , through=> :friendships
  attr_accessor :email, :password
  before_save :encrypt_password
  after_save :clear_password
  validates_prescence_of :first_name
  validates_prescence_of :last_name
  validates :password, presence: true, length: { minimum: 6 }
  validates_prescence_of :email
  validates_uniqueness_of :email , uniqueness: { case_sensitive: false }
  validates_prescence_of :gender
  validate :that_born_on_is_not_in_the_future



  def self.authenticate(email, password)
      user = find_by_email(email)
      if ((user && user.password_hash) == (BCrypt::Engine.hash_secret(password, user.salt)))
       user
      else
        nil
      end
  end

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

  def clear_password
    self.password = nil
  end


  def that_born_on_is_not_in_the_future
      self.errors.add :birth_date, 'is in the future' \
  unless self.birth_date <= Date.today end


  end

Thanks in advance.