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.