MONGOID-5797 Fix accessing parent when projected (mongodb#5847) (mongodb#5850)

Author: comandeo-mongo

lib/mongoid/association/accessors.rb

@@ -115,7 +115,11 @@ def get_relation(name, association, object, reload = false)
         # during binding or when cascading callbacks. Whenever we retrieve
         # associations within the codebase, we use without_autobuild.
         if !without_autobuild? && association.embedded? && attribute_missing?(field_name)
-          raise ActiveModel::MissingAttributeError, "Missing attribute: '#{field_name}'"
+          # We always allow accessing the parent document of an embedded one.
+          try_get_parent = association.is_a?(
+                             Mongoid::Association::Embedded::EmbeddedIn
+                           ) && field_name == association.key
+          raise ActiveModel::MissingAttributeError, "Missing attribute: '#{field_name}'" unless try_get_parent
         end
 
         if !reload && (value = ivar(name)) != false

spec/mongoid/association/embedded/embeds_many_query_spec.rb

@@ -27,6 +27,10 @@
       expect(legislator.attributes.keys).to eq(['_id', 'a'])
     end
 
+    it 'allows accessing the parent' do
+      expect { legislator.congress }.not_to raise_error
+    end
+
     context 'when using only with $' do
       before do
         Patient.destroy_all