Fix autosaving for accepts_nested_attributes_for (only for non-embedded)

lib/mongoid/attributes/nested.rb

@@ -83,11 +83,20 @@ def autosave_nested_attributes(association)
           # marked as autosave despite the fact that the option isn't present. Because the method
           # Association#autosave? is implemented by checking the autosave option, this is the most
           # straightforward way to mark it.
-          if association.autosave? || (association.options[:autosave].nil? && !association.embedded?)
+          #
+          # If autosave option for association is present but is set as false, then association will
+          # be checked if it is one of the values of `nested_attributes`. If existing, then association
+          # will be marked as autosave.
+          is_nested = is_association_nested_attribute?(association)
+          if association.autosave? || ((association.options[:autosave].nil? || is_nested) && !association.embedded?)
             association.options[:autosave] = true
             Association::Referenced::AutoSave.define_autosave!(association)
           end
         end
+
+        def is_association_nested_attribute?(association)
+          !!nested_attributes[association.name.to_s + '_attributes']
+        end
       end
     end
   end

spec/mongoid/attributes/nested_spec.rb

@@ -49,6 +49,10 @@
 
     context "when autosave is explicitly false" do
 
+      let(:account) do
+        Account.new
+      end
+
       before do
         Account.accepts_nested_attributes_for :alerts
       end
@@ -62,8 +66,12 @@
         Account.reflect_on_association(:alerts)
       end
 
-      it "keeps autosave set to false" do
-        expect(association).to_not be_autosave
+      it "sets autosave to true" do
+        expect(association).to be_autosave
+      end
+
+      it "autosaves if the association is not embedded" do
+        expect(account).to respond_to(:autosave_documents_for_alerts)
       end
     end
   end