Fix embedded elements memoization following #changed?

- Fix #3991
- Close mongodb#3992

CHANGELOG.md

@@ -49,6 +49,9 @@ For instructions on upgrading to newer versions, visit
 
 * \#3993 Fixes issue where `dup`/`clone` fails for embedded documents that use store_as without using Mongoid::Atributes::Dynamic
 
+* \#3991 Fixed emebdded documents not flagging as changed after calling #changed? and modifying the
+child elements.
+
 * \#3874 Adding snapshot option to context.
 
 * \#3868 Loading models in rake tasks now expands the rails path.

lib/mongoid/relations/embedded/many.rb

@@ -84,6 +84,7 @@ def build(attributes = {}, type = nil)
           doc.apply_post_processed_defaults
           yield(doc) if block_given?
           doc.run_callbacks(:build) { doc }
+          base._reset_memoized_children!
           doc
         end
         alias :new :build

lib/mongoid/traversable.rb

@@ -124,6 +124,21 @@ def reset_persisted_children
         child.move_changes
         child.new_record = false
       end
+      _reset_memoized_children!
+    end
+
+    # Resets the memoized children on the object. Called internally when an
+    # embedded array changes size.
+    #
+    # @api semiprivate
+    #
+    # @example Reset the memoized children.
+    #   document._reset_memoized_children!
+    #
+    # @return [ nil ] nil.
+    #
+    # @since 5.0.0
+    def _reset_memoized_children!
       @__children = nil
     end
 

spec/mongoid/changeable_spec.rb

@@ -811,6 +811,58 @@
       end
     end
 
+    context "when changed? has been called before child elements size change" do
+
+      let(:person) do
+        Person.create
+      end
+
+      let(:address) do
+        person.addresses.create(street: "hobrecht")
+      end
+
+      let!(:location) do
+        address.locations.create(name: "home")
+      end
+
+      before do
+        person.changed?
+      end
+
+      context "when adding via new" do
+
+        before do
+          address.locations.new
+        end
+
+        it "returns true" do
+          expect(person).to be_changed
+        end
+      end
+
+      context "when adding via build" do
+
+        before do
+          address.locations.build
+        end
+
+        it "returns true" do
+          expect(person).to be_changed
+        end
+      end
+
+      context "when adding via create" do
+
+        before do
+          address.locations.create
+        end
+
+        it "returns false" do
+          expect(person).to_not be_changed
+        end
+      end
+    end
+
     context "when a deeply embedded child has changed" do
 
       let(:person) do