Why Rails 5 uses ApplicationRecord instead of ActiveRecord::Base?

While it may seem the same in basic Rails applications, there actually is an important difference once you begin to use rails engines, plugins / gems or direct methods from ActiveRecord::Base.

• ActiveRecord::Base.include(MyFeatures) mixes in the features directly into ActiveRecord::Base and it is present there forever for all later uses of ActiveRecord::Base (it cannot be "unmixed") and there is no way to get the original ActiveRecord::Base anymore in any code after the include. This can easily lead to problems if some of the mixed in feature changed the default ActiveRecord behavior or if e.g. two engines / gems tried to include same-named methods.

• On the other hand, the ApplicationRecord approach makes the features present only for the classes (models) that inherit from it, other classes, as well as direct use of ActiveRecord::Base stay pristine, uncluttered by the module features.

This is especially important when engines or rails plugins are used as it allows them to separate their own model logic from the main application's model logic which was not possible before ApplicationRecord.

All of this is also nicely described in this blog post and this github comment.

This is to expand on @BoraMa's answer, and to, hopefully, clear up some confusion around ActiveRecord::Base.abstract_class.

ActiveRecord::Base.abstract_class goes back to at least Rails 3.2.0 (http://api.rubyonrails.org/v3.2.0/classes/ActiveRecord/Inheritance/ClassMethods.html), which was released on January 20, 2012.

Rails 4.0.0 improved the documentation: http://api.rubyonrails.org/v4.0.0/classes/ActiveRecord/Inheritance/ClassMethods.html

So, to everyone who thinks ApplicationRecord is radically new, it's not. It is an improvement, not a breaking change. Nothing was added to ActiveRecord::Base to make this work.

I did the same thing on a Rails 4.2.6 project because the models used UUIDs for ids instead of integers, and this required a change to the default ORDER BY. So, instead of using copy-paste or a concern, I went with inheritance using a UuidModel class and self.abstract_class = true.