Rspec (for Michael Hartl example) for the definition of relationship following users

I'm trying to make a spec test for the MHartl's example. As it follows bellow my difficult is for #Follows a user.

class User < ActiveRecord::Base

  # Associations
  has_many :active_relationships, class_name:  "Relationship",
                                  foreign_key: "follower_id",
                                  dependent:   :destroy
  has_many :following, through: :active_relationships, source: :followed
  has_many :followers, through: :passive_relationships, source: :follower


  # Follows a user.
  def follow(other_user)
    active_relationships.create(followed_id: other_user.id)
  end

For my spec:

require 'rails_helper'

RSpec.describe User, :type => :model do
  let(:user) { build(:user) }

  describe 'Validations' do
    it 'has a valid factory' do
      expect(user).to be_valid
    end

  describe 'following utility methods' do
    it "user starts to follow other user" do
      user.follow(:other_user)
      user.following?(:other_user).should be_valid
    end
  end
end

My failures :

Failures:

  1) User following utility methods user starts to follow other user
     Failure/Error: active_relationships.create(followed_id: other_user.id)

     NoMethodError:
       undefined method `id' for :other_user:Symbol

If you need more info about this post, please fell free to ask! and please tell me were can I learn more about this specs tests. thanks for your help :)