undefined method `users' for Tests - ruby-on-rails

Question

I'm trying to write a simple test but my application is not detecting 'users' keyword when I'm using it in my Test functions:
post_controller_test.rb:
require 'test_helper'
class PostsControllerTest < ActionController::TestCase
# test "the truth" do
# assert true
# end
test "should fetch facebook post" do
sign_in users(:one)
get(:save_posts_from_facebook_page,{'id'=>'my_id'},{'access_token'=>Rails.application.secrets.fb_access_token})
assert_response :success
end
end
test_helper.rb:
ENV['RAILS_ENV'] ||= 'test'
require File.expand_path('../../config/environment', __FILE__)
require 'rails/test_help'
class ActiveSupport::TestCase
include Devise::TestHelpers
# Add more helper methods to be used by all tests here...
end
users.yml:
# Read about fixtures at http://api.rubyonrails.org/classes/ActiveRecord/FixtureSet.html
one:
email: hello#testing.com
encrypted_password: <%= Devise.bcrypt(User,'password') %>
links:
causes:
two:
email: MyString
password: MyString
links:
causes:
Output of rake/Error:
Run options: --seed 42684
# Running:
E
Finished in 0.010275s, 97.3279 runs/s, 0.0000 assertions/s.
1) Error:
PostsControllerTest#test_should_fetch_facebook_post:
NoMethodError: undefined method `users' for #<PostsControllerTest:0x000001042152f0>
test/controllers/posts_controller_test.rb:9:in `block in <class:PostsControllerTest>'
1 runs, 0 assertions, 0 failures, 1 errors, 0 skips
Update:
Note that I'm using MongoId.
When I add 'fixtures :all' to my test_helper.rb, I get:
rake
rake aborted!
NoMethodError: undefined method `fixtures' for ActiveSupport::TestCase:Class
/Users/gautambajaj/My Stuff/FreeFromBorders/f2b_website/test/test_helper.rb:7:in `<class:TestCase>'
/Users/gautambajaj/My Stuff/FreeFromBorders/f2b_website/test/test_helper.rb:5:in `<top (required)>'
/Users/gautambajaj/.rvm/gems/ruby-2.1.2/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:274:in `require'
/Users/gautambajaj/.rvm/gems/ruby-2.1.2/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:274:in `block in require'
/Users/gautambajaj/.rvm/gems/ruby-2.1.2/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:240:in `load_dependency'
/Users/gautambajaj/.rvm/gems/ruby-2.1.2/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:274:in `require'
/Users/gautambajaj/My Stuff/FreeFromBorders/f2b_website/test/controllers/posts_controller_test.rb:1:in `<top (required)>'
/Users/gautambajaj/.rvm/gems/ruby-2.1.2/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:274:in `require'
/Users/gautambajaj/.rvm/gems/ruby-2.1.2/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:274:in `block in require'
/Users/gautambajaj/.rvm/gems/ruby-2.1.2/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:240:in `load_dependency'
/Users/gautambajaj/.rvm/gems/ruby-2.1.2/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:274:in `require'
/Users/gautambajaj/.rvm/gems/ruby-2.1.2/gems/railties-4.2.1/lib/rails/test_unit/sub_test_task.rb:114:in `block (3 levels) in define'
/Users/gautambajaj/.rvm/gems/ruby-2.1.2/gems/railties-4.2.1/lib/rails/test_unit/sub_test_task.rb:114:in `each'
/Users/gautambajaj/.rvm/gems/ruby-2.1.2/gems/railties-4.2.1/lib/rails/test_unit/sub_test_task.rb:114:in `block (2 levels) in define'
/Users/gautambajaj/.rvm/gems/ruby-2.1.2/gems/railties-4.2.1/lib/rails/test_unit/sub_test_task.rb:113:in `each'
/Users/gautambajaj/.rvm/gems/ruby-2.1.2/gems/railties-4.2.1/lib/rails/test_unit/sub_test_task.rb:113:in `block in define'
/Users/gautambajaj/.rvm/gems/ruby-2.1.2/gems/railties-4.2.1/lib/rails/test_unit/sub_test_task.rb:20:in `invoke_rake_task'
/Users/gautambajaj/.rvm/gems/ruby-2.1.2/gems/railties-4.2.1/lib/rails/test_unit/testing.rake:8:in `block in <top (required)>'
Tasks: TOP => test:run
(See full trace by running task with --trace)

Answer 1

In your test_helper.rb file, you need to add fixtures :all:
class ActiveSupport::TestCase
# Setup all fixtures in test/fixtures/*.yml for all tests in alphabetical order.
fixtures :all
# rest of the codes in this file
end
Once you add that line in your test_helper.rb file as shown above, your test will pass.
Update
As Sergio Tulentsev mentioned in the comment, Mongodb doesn't support transactions, so transactional fixtures can't be used in your case. Take a look at this answer which states that and also this google group discussion as well.
I would recommend you to use factory_girl instead, which supports mongoid and is pretty awesome.