So a NullRelation (Relation#none) is chainable with database methods.

The #relation method gets called in four places and the return value was instantly cloned in three of them. The only place that did not clone was ActiveRecord::Scoping::Default::ClassMethods#unscoped. This introduced a bug described in #5667 and should really clone the relation, too. This means all four places would clone the relation, so it doesn't make a lot of sense caching it in the first place.

The four places with calls to relations are:

activerecord/lib/active_record/scoping/default.rb:110:in `block in build_default_scope'"
activerecord/lib/active_record/scoping/default.rb:42:in `unscoped'"
activerecord/lib/active_record/scoping/named.rb:38:in `scoped'"
activerecord/lib/active_record/scoping/named.rb:52:in `scope_attributes'"

And added NullRelation class implementing the null object pattern for the `Relation` class.

See the CHANGELOG for details.

Fixes #950.

This reverts commit c99d507f.

For consistency with find_or_initialize_by. Also remove first_or_build
alias.

This reverts commit 0df27c98.

Reverted due to failing test, see #2845.

Replace split on comma with a regexp that will reverse all ASC/DESC specifically

Last two asserts in this test assume that all_comments are ordered by posts.id and then by comments.id therefore additional ordering is added. Without it test was failing on Oracle which returned results in different order.

This let's you write things like:

    User.where(:first_name => "Scarlett").first_or_create!(:last_name => "Johansson", :hot => true)

Related to #2420.

This reverts commit 9d396ee8, reversing
changes made to fa2bfd83.

Reason: the change broke the build.

Simplify the test by using id and name. `id` will be the only real sort criteria in any case as it's unique.