diff --git a/Gemfile b/Gemfile
index 3d60024..260cb7a 100644
--- a/Gemfile
+++ b/Gemfile
@@ -43,7 +43,7 @@ else
 end
 
 if ENV['DB'] =~ /mongodb/
-  gem 'mongoid', '~> 5.0.0', require: false
+  gem 'mongoid', '~> 4.0.0', require: false
 end
 
 # Removed from Ruby 2.2 but needed for testing Rails 3.x.
diff --git a/spec/support/schema.rb b/spec/support/schema.rb
index d46b6ce..f5c46a6 100644
--- a/spec/support/schema.rb
+++ b/spec/support/schema.rb
@@ -25,7 +25,7 @@ else
 end
 
 class Person < ActiveRecord::Base
-  if ActiveRecord::VERSION::MAJOR == 3
+  if ::ActiveRecord::VERSION::MAJOR == 3
     default_scope order('id DESC')
   else
     default_scope { order(id: :desc) }
@@ -125,7 +125,7 @@ class Article < ActiveRecord::Base
 
   alias_attribute :content, :body
 
-  if ActiveRecord::VERSION::STRING >= '3.1'
+  if ::ActiveRecord::VERSION::STRING >= '3.1'
     default_scope { where("'default_scope' = 'default_scope'") }
   else # Rails 3.0 does not accept a block
     default_scope where("'default_scope' = 'default_scope'")