Chaining named scope is no longer leaking to class level querying methods

Active Record uses `scoping` to delegate to named scopes from relations
for propagating the chaining source scope. It was needed to restore the
source scope in named scopes, but it was caused undesired behavior that
pollute all class level querying methods.

Example:

```ruby
class Topic < ActiveRecord::Base
  scope :toplevel, -> { where(parent_id: nil) }
  scope :children, -> { where.not(parent_id: nil) }
  scope :has_children, -> { where(id: Topic.children.select(:parent_id)) }
end

# Works as expected.
Topic.toplevel.where(id: Topic.children.select(:parent_id))

# Doesn't work due to leaking `toplevel` to `Topic.children`.
Topic.toplevel.has_children
```

Since #29301, the receiver in named scopes has changed from the model
class to the chaining source scope, so the polluting class level
querying methods is no longer required for that purpose.

Fixes #14003.

activerecord/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Chaining named scope is no longer leaking to class level querying methods.
+
+    Fixes #14003.
+
+    *Ryuta Kamizono*
+
 *   Allow applications to automatically switch connections.
 
     Adds a middleware and configuration options that can be used in your

activerecord/lib/active_record/relation.rb

@@ -312,7 +312,7 @@ module ActiveRecord
     # Please check unscoped if you want to remove all previous scopes (including
     # the default_scope) during the execution of a block.
     def scoping
-      @delegate_to_klass ? yield : klass._scoping(self) { yield }
+      @delegate_to_klass && klass.current_scope ? yield : klass._scoping(self) { yield }
     end
 
     def _exec_scope(*args, &block) # :nodoc:

activerecord/lib/active_record/relation/spawn_methods.rb

@@ -8,7 +8,7 @@ module ActiveRecord
   module SpawnMethods
     # This is overridden by Associations::CollectionProxy
     def spawn #:nodoc:
-      @delegate_to_klass ? klass.all : clone
+      @delegate_to_klass && klass.current_scope ? klass.all : clone
     end
 
     # Merges in the conditions from <tt>other</tt>, if <tt>other</tt> is an ActiveRecord::Relation.

activerecord/lib/active_record/scoping.rb

@@ -23,11 +23,11 @@ module ActiveRecord
         current_scope
       end
 
-      private
-        def current_scope(skip_inherited_scope = false)
-          ScopeRegistry.value_for(:current_scope, self, skip_inherited_scope)
-        end
+      def current_scope(skip_inherited_scope = false)
+        ScopeRegistry.value_for(:current_scope, self, skip_inherited_scope)
+      end
 
+      private
         def current_scope=(scope)
           ScopeRegistry.set_value_for(:current_scope, self, scope)
         end
@@ -84,7 +84,7 @@ module ActiveRecord
         base = model.base_class
         while klass <= base
           value = @registry[scope_type][klass.name]
-          return value if value
+          return value || nil unless value.nil?
           klass = klass.superclass
         end
       end

activerecord/lib/active_record/scoping/named.rb

@@ -180,7 +180,8 @@ module ActiveRecord
 
           if body.respond_to?(:to_proc)
             singleton_class.define_method(name) do |*args|
-              scope = all._exec_scope(*args, &body)
+              scope = all
+              scope = _scoping(false) { scope._exec_scope(*args, &body) }
               scope = scope.extending(extension) if extension
               scope
             end

activerecord/test/cases/scoping/named_scoping_test.rb

@@ -447,6 +447,16 @@ class NamedScopingTest < ActiveRecord::TestCase
     assert_equal [posts(:sti_comments)], Post.with_special_comments.with_post(4).to_a.uniq
   end
 
+  def test_chaining_doesnt_leak_conditions_to_another_scopes
+    expected = Topic.where(approved: false).where(id: Topic.children.select(:parent_id))
+    assert_equal expected.to_a, Topic.rejected.has_children.to_a
+  end
+
+  def test_nested_scoping
+    expected = Reply.approved
+    assert_equal expected.to_a, Topic.rejected.nested_scoping(expected)
+  end
+
   def test_scopes_batch_finders
     assert_equal 4, Topic.approved.count
 

activerecord/test/models/topic.rb

@@ -10,6 +10,9 @@ class Topic < ActiveRecord::Base
   scope :approved, -> { where(approved: true) }
   scope :rejected, -> { where(approved: false) }
 
+  scope :children, -> { where.not(parent_id: nil) }
+  scope :has_children, -> { where(id: Topic.children.select(:parent_id)) }
+
   scope :scope_with_lambda, lambda { all }
 
   scope :by_lifo, -> { where(author_name: "lifo") }
@@ -88,6 +91,10 @@ class Topic < ActiveRecord::Base
     write_attribute(:approved, val)
   end
 
+  def self.nested_scoping(scope)
+    scope.base
+  end
+
   private
 
     def default_written_on