Simplify/fix implementation of default scopes

The previous implementation was necessary in order to support stuff
like:

    class Post < ActiveRecord::Base
      default_scope where(published: true)
      scope :ordered, order("created_at")
    end

If we didn't evaluate the default scope at the last possible moment
before sending the SQL to the database, it would become impossible to
do:

    Post.unscoped.ordered

This is because the default scope would already be bound up in the
"ordered" scope, and therefore wouldn't be removed by the
"Post.unscoped" part.

In 4.0, we have deprecated all "eager" forms of scopes. So now you must
write:

    class Post < ActiveRecord::Base
      default_scope { where(published: true) }
      scope :ordered, -> { order("created_at") }
    end

This prevents the default scope getting bound up inside the "ordered"
scope, which means we can now have a simpler/better/more natural
implementation of default scoping.

A knock on effect is that some things that didn't work properly now do.
For example it was previously impossible to use #except to remove a part
of the default scope, since the default scope was evaluated after the
call to #except.

1 files changed, 6 insertions, 10 deletions

diff --git a/activerecord/lib/active_record/scoping/named.rb b/activerecord/lib/active_record/scoping/named.rb
index da73bead32..e7d5e6ce84 100644
--- a/activerecord/lib/active_record/scoping/named.rb
+++ b/activerecord/lib/active_record/scoping/named.rb
@@ -25,22 +25,18 @@ module ActiveRecord
           if current_scope
             current_scope.clone
           else
-            scope = relation
-            scope.default_scoped = true
-            scope
+            default_scoped
           end
         end
 
+        def default_scoped # :nodoc:
+          relation.merge(build_default_scope)
+        end
+
         # Collects attributes from scopes that should be applied when creating
         # an AR instance for the particular class this is called on.
         def scope_attributes # :nodoc:
-          if current_scope
-            current_scope.scope_for_create
-          else
-            scope = relation
-            scope.default_scoped = true
-            scope.scope_for_create
-          end
+          all.scope_for_create
         end
 
         # Are there default attributes associated with this scope?