Error creating Feed with Ruby on Rails

class User
  include Mongoid::Document
  include Mongoid::Timestamps
  include ActiveModel::SecurePassword

  embeds_many :microposts

  
  field :name, type: String
  field :email, type: String
  field :password_digest, type: String
  field :remember_digest, type: String
  field :admin, type: Boolean, default: false
  field :activation_digest, type: String
  field :activated, type: Boolean, default: false
  field :activated_at, type: DateTime
  field :reset_digest, type: String
  field :reset_sent_at, type: DateTime
  has_and_belongs_to_many :following, class_name: 'User', inverse_of: :followers
  has_and_belongs_to_many :followers, class_name: 'User', inverse_of: :following
  
  

  attr_accessor :remember_token, :activation_token, :reset_token, :reset_token
  before_save   :downcase_email
  before_create :create_activation_digest
  validates :name,  presence: true, length: { maximum: 50 }
  VALID_EMAIL_REGEX = /\A[\w+\-.]+@[a-z\d\-.]+\.[a-z]+\z/i
  validates :email, presence: true, length: { maximum: 255 }, format: { with: VALID_EMAIL_REGEX },uniqueness: { case_sensitive: false }
  
  has_secure_password
  validates :password, presence: true, length: { minimum: 5 }, allow_nil: true
  
  def feed
    posts = self.microposts
    self.following.each do |user_siguiendo|
      posts = posts + user_siguiendo.microposts
    end
    if posts.count >= 2 then posts = posts.sort { |x, y| y.created_at <=> x.created_at } end
    return posts
  end
  
    # Returns the hash digest of the given string.
    def User.digest(string)
      cost = ActiveModel::SecurePassword.min_cost ? BCrypt::Engine::MIN_COST : BCrypt::Engine.cost
      BCrypt::Password.create(string, cost: cost)
    end
  
    # Returns a random token.
    def User.new_token
      SecureRandom.urlsafe_base64
    end
    
    # Remembers a user in the database for use in persistent sessions.
    def remember
      self.remember_token = User.new_token
      update_attribute(:remember_digest, User.digest(remember_token))
    end

    # Returns true if the given token matches the digest.
  def authenticated?(attribute, token)
    digest = send("#{attribute}_digest")
    return false if digest.nil?
    BCrypt::Password.new(digest).is_password?(token)
  end
    
  # Forgets a user.
  def forget
    update_attribute(:remember_digest, nil)
  end
  
  # Returns true if the given token matches the digest.
  def authenticated?(attribute, token)
    digest = send("#{attribute}_digest")
    return false if digest.nil?
    BCrypt::Password.new(digest).is_password?(token)
  end
  
  # Activates an account.
  def activate
    update_attribute(:activated,    true)
    update_attribute(:activated_at, Time.zone.now)
  end

  # Sends activation email.
  def send_activation_email
    UserMailer.account_activation(self).deliver_now
  end
  
  # Sets the password reset attributes.
  def create_reset_digest
    self.reset_token = User.new_token
    update_attribute(:reset_digest,  User.digest(reset_token))
    update_attribute(:reset_sent_at, Time.zone.now)
  end
  
  # Sends password reset email.
  def send_password_reset_email
    UserMailer.password_reset(self).deliver_now
  end
  
  # Returns true if a password reset has expired.
  def password_reset_expired?
    reset_sent_at < 2.hours.ago
  end
  

  # Defines a proto-feed.
  # See "Following users" for the full implementation.
  #def feed
   # self.microposts.all
  #end
  
  # Returns a user's status feed.
  #def feed
    #following_ids = "SELECT followed_id FROM relationships WHERE follower_id = :user_id"
    #Micropost.where("user_id IN (#{following_ids}) OR user_id = :user_id", user_id: id)
  #end
  
  # Follows a user.
  def follow(other_user)
    self.following << other_user
    other_user.followers << self
  end

  # Unfollows a user.
  def unfollow(other_user)
    self.following.delete(other_user)
    other_user.followers.delete(self)
  end

  # Returns true if the current user is following the other user.
  def following?(other_user)
    self.following.include?(other_user)
  end
  
  private
  
  # Converts email to all lower-case.
    def downcase_email
      self.email = email.downcase
    end
    
    # Creates and assigns the activation token and digest.
    def create_activation_digest
      self.activation_token  = User.new_token
      self.activation_digest = User.digest(activation_token)
    end

end #class User

class StaticPagesController < ApplicationController
	
  def home
    if logged_in?
      @feed_items = current_user.feed.paginate(page: params[:page])
      @micropost  = current_user.microposts.build
    end
  end
  
  def help
  end

  def contact
  end

  def about
  end
end