MONGOID-5186 .with_scope should restore previous scope (#5127)

Co-authored-by: Oleg Pudeyev <code@olegp.name>

Author: p-mongo

docs/reference/queries.txt

@@ -1577,6 +1577,7 @@ To have Mongoid raise an error when a scope would overwrite an existing class
 method, set the ``scope_overwrite_exception`` :ref:`configuration option
 <configuration-options>` to ``true``.
 
+
 Default Scopes
 --------------
 
@@ -1705,6 +1706,35 @@ that would affect its visibility within the scoped association.
   label.reload.bands # []
 
 
+Runtime Default Scope Override
+------------------------------
+
+You can use the ``with_scope`` method to change the default scope in a block
+at runtime:
+
+.. code-block:: ruby
+
+  class Band
+    include Mongoid::Document
+    field :country, type: String
+    field :genres, type: Array
+
+    scope :english, ->{ where(country: "England") }
+  end
+
+  criteria = Band.with_scope(Band.english) do
+    Band.all
+  end
+  
+  criteria
+  # => 
+  # #<Mongoid::Criteria
+  #   selector: {"country"=>"England"}
+  #   options:  {}
+  #   class:    Band
+  #   embedded: false>
+
+
 Class Methods
 -------------
 

lib/mongoid/scopable.rb

@@ -203,11 +203,12 @@ def with_default_scope
       #
       # @return [ Criteria ] The yielded criteria.
       def with_scope(criteria)
+        previous = Threaded.current_scope(self)
         Threaded.set_current_scope(criteria, self)
         begin
           yield criteria
         ensure
-          Threaded.set_current_scope(nil, self)
+          Threaded.set_current_scope(previous, self)
         end
       end
 

spec/mongoid/scopable_spec.rb

@@ -1126,6 +1126,26 @@ class << Band
         expect(Mongoid::Threaded.current_scope(Band)).to be_nil
       end
     end
+
+    context 'when nesting with_scope calls' do
+      let(:c1) { Band.where(active: true) }
+      let(:c2) { Band.where(active: false) }
+
+      it 'restores previous scope' do
+        Band.with_scope(c1) do |crit|
+          Band.with_scope(c2) do |crit2|
+            Mongoid::Threaded.current_scope(Band).selector.should == {
+              'active' => true,
+              '$and' => ['active' => false],
+            }
+          end
+
+          Mongoid::Threaded.current_scope(Band).selector.should == {
+            'active' => true,
+          }
+        end
+      end
+    end
   end
 
   describe ".without_default_scope" do