We discussed that the auto explain feature is rarely used. This PR removes only the automatic explain. You can still display the explain output for any given relation using `ActiveRecord::Relation#explain`. As a side-effect this should also fix the connection problem during asset compilation (#9385). The auto explain initializer in the `ActiveRecord::Railtie` forced a connection.
Most apps upgrading from 3.x will have options for mass assigment in their application.rb and environments/*.rb config files. Rather than just raising a NoMethodError when copying the config, this commit adds a warning message until either the protected_attributes gem is installed or the relevant config options are removed.
In the end I think the pain of implementing this seamlessly was not worth the gain provided. The intention was that it would allow plain ruby objects that might not live in your main application to be subclassed and have persistence mixed in. But I've decided that the benefit of doing that is not worth the amount of complexity that the implementation introduced.
…mespaces The new option allows any Ruby namespace to be registered and set up for eager load. We are effectively exposing the structure existing in Rails since v3.0 for all developers in order to make their applications thread-safe and CoW friendly.