Part of the code for my blog

post.rb

class Post < ActiveRecord::Base
  belongs_to :user, -> { where admin: true }
  default_scope -> { order(created_at: :desc) }
  validates :user_id, presence: true
  validates :title, presence: true
  validates :content, presence: true
end

post_factory.rb

# actually named spec/models/post.rb, but I can't have duplicate file names here
FactoryGirl.define do
  factory :post do |p|
    p.title Faker::Lorem.sentence(4)
    p.content Faker::Lorem.sentence(100)
  end
end

post_spec.rb

require 'rails_helper'

RSpec.describe Post, type: :model do
  before do
    @user_a = FactoryGirl.create(:user, username: 'michael', email: 'michael@example.com', admin: true)
    @user_b = FactoryGirl.create(:user, username: 'joe', email: 'joe@example.com', admin: false)
  end
  it 'has a valid factory' do
    FactoryGirl.create(:post, user: @user_a).should be_valid
  end
  it 'is invalid without a title' do
    FactoryGirl.build(:post, title: "", user: @user_a).should_not be_valid
  end
  it 'is invalid without content' do
    FactoryGirl.build(:post, content: "", user: @user_a).should_not be_valid
  end
  it 'is invalid without a user' do
    FactoryGirl.build(:post, user: nil)
  end
  it 'orders by highest id' do
    post_a = @user_a.posts.create!(content: Faker::Lorem.sentence(100), title: Faker::Lorem.sentence(4))
    post_b = @user_a.posts.create!(content: Faker::Lorem.sentence(100), title: Faker::Lorem.sentence(4))
    expect(Post.all).to eq([post_b, post_a])
  end
  # test that fails
  it 'should not be created if the user is not admin' do
    FactoryGirl.build(:post, user: @user_b).should_not be_valid
  end
end

posts_controller.rb

class PostsController < ApplicationController
  before_action :admin_user, only: [:new, :create, :edit, :update, :destroy]

  def index
    @posts = Post.paginate(page: params[:page])
  end

  def show
    @post = Post.find(params[:id])
  end

  def new
    @post = Post.new
  end

  def create
    @post = current_user.posts.build(post_params)
    if @post.save
      flash[:success] = "Blog post created!"
      redirect_to blog_url 
    else
      render 'new' 
    end
  end

  def edit
    @post = Post.find(params[:id])
  end

  def update
    @post = Post.find(params[:id])
    if @post.update_attributes(post_params)
      flash[:success] = "Blog post updated!"
      redirect_to @post
    else
      redirect_to action: :edit 
    end
  end

  def destroy
    Post.find(params[:id]).destroy
    flash[:success] = "Blog post deleted"
    redirect_to blog_url
  end

  private
    def post_params
      params.require(:post).permit(:title, :content)
    end

    def admin_user
      redirect_to(blog_url) unless current_user.admin?
    end
end

user.rb

FactoryGirl.define do
  factory :user do |u|
    u.password 'test_password'
    u.password_confirmation 'test_password'
  end
end