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.
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.
I think it's going to be too much pain to try to transition the
:active_record load hook from executing against Base to executing
against Model.
For example, after Model is included in Base, and modules included in
Model will no longer get added to the ancestors of Base.
So plugins which wish to be compatible with both Model and Base should
use the :active_record_model load hook which executes *before* Base gets
loaded.
In general, ActiveRecord::Model is an advanced feature at the moment and
probably most people will continue to inherit from ActiveRecord::Base
for the time being.
The problem: We need to be able to specify configuration in a way that
can be inherited to models that include ActiveRecord::Model. So it is
no longer sufficient to put 'top level' config on ActiveRecord::Base,
but we do want configuration specified on ActiveRecord::Base and
descendants to continue to work.
So we need something like class_attribute that can be defined on a
module but that is inherited when ActiveRecord::Model is included.
The solution: added ActiveModel::Configuration module which provides a
config_attribute macro. It's a bit specific hence I am not putting this
in Active Support or making it a 'public API' at present.
This commit vastly reduces the impact of auto
explain logging when enabled, while keeping
a negligible cost when disabled.
The first implementation was based on the idea
of subscribing to "sql.active_record" when
needed, and unsubscribing once done. This is
the idea behind AR::Relation#explain. Subscribe,
collect, unsubscribe.
But with the current implementation of notifications
unsubscribing is costly, because it wipes an internal
cache and that puts a penalty on the next event.
So we are switching to an approach where a long-running
subscriber is listening. Instead of collecting the
queries with a closure in a dedicated subscriber, now
we setup a thread local.
If the feature is disabled by setting the threshold
to nil, the subscriber will call a method that does
nothing. That's totally cheap.
Rationale: As discussed with José and Jon, this convenience
shortcut is not clearly justified and it could let the user
thing the disabled EXPLAINs are related to the model instance
rather than being globally disabled.
