A series of commits relating to preloading and scopes caused a regression.
Cloning the relation calls initialize_copy which resets a number of instance
variables to nil. Without this the scope thinks that it is already loaded
when it is called again.
Reverts the following commits:
13f1401a6c8491740ca5dffbb521a0Fixes#6575, #6576 & #6577
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'"
Conflicts:
activerecord/lib/active_record/core.rb
