This is the 'top level' connection, inherited by any models that include
ActiveRecord::Model or inherit from ActiveRecord::Base.

it clear which methods are synchronized.

the following code is deprecated:

Thread.new { Post.find(1) }.join

It should be changed to close the database connection at the end of
the thread:

Thread.new {
  Post.find(1)
  Post.connection.close
}.join

Only people who spawn threads in their application code need to worry
about this change.

pool.

use thread locals and an instance variable within QueryCache#BodyProxy to maintain appropriate linkage with AR database connection across threads

Patch for issue #2820

Make it the responsibility of the connection to hold onto an ARel visitor for generating SQL. This improves the code architecture generally, and solves some problems with marshalling. Adapter authors please take note: you now need to define an Adapter.visitor_for method, but it degrades gracefully with a deprecation warning for now.

Fix inconsistencies by being polite to the wrapped body. Needed for Rack::Sendfile to function properly. See issue #1761.

Allow the connection pool's #table_exists? method to give the connections #table_exists? method a chance.