MONGOID-5206 fix bug where embedded document is not re-embedded

lib/mongoid/association/embedded/embeds_one/proxy.rb

@@ -53,6 +53,29 @@ def substitute(replacement)
                 # The associated object will be replaced by the below update if non-nil, so only
                 # run the callbacks and state-changing code by passing persist: false in that case.
                 _target.destroy(persist: !replacement) if persistable?
+
+                # A little explanation on why this is needed... Say we have three assignments:
+                #
+                # canvas.palette = palette
+                # canvas.palette = nil
+                # canvas.palette = palette
+                # Where canvas embeds_one palette.
+                #
+                # Previously, what was happening was, on the first assignment,
+                # palette was considered a "new record" (new_record?=true) and
+                # thus palette was being inserted into the database. However,
+                # on the third assignment, we're trying to reassign the palette,
+                # palette is no longer considered a new record, because it had
+                # been inserted previously. This is not exactly accurate,
+                # because the second assignment ultimately removed the palette
+                # from the database, so it needs to be reinserted. Since the
+                # palette's new_record is false, Mongoid ends up "updating" the
+                # document, which doesn't reinsert it into the database.
+                #
+                # The change I introduce here, respecifies palette as a "new
+                # record" when it gets removed from the database, so if it is
+                # reassigned, it will be reinserted into the database.
+                _target.new_record = true
               end
               unbind_one
               return nil unless replacement

spec/mongoid/association/accessors_spec.rb

@@ -166,6 +166,19 @@
           end
         end
       end
+
+      context "when the association is set to nil first" do
+
+        let!(:name) do
+          person.build_name
+        end
+
+        it "returns true" do
+          person.name = nil
+          person.name = name
+          expect(person).to have_name
+        end
+      end
     end
 
     context "when the association is an embeds many" do

spec/mongoid/reloadable_spec.rb

@@ -288,6 +288,29 @@
       end
     end
 
+    context "when embedded documents are unasssigned and reassigned" do
+
+      let(:palette) do
+        Palette.new
+      end
+
+      let(:canvas) do
+        Canvas.create!
+      end
+
+      before do
+        canvas.palette = palette
+        canvas.palette = nil
+        canvas.palette = palette
+        canvas.save!
+        canvas.reload
+      end
+
+      it "reloads the embedded document correctly" do
+        expect(canvas.palette).to eq(palette)
+      end
+    end
+
     context "with relational associations" do
 
       let(:person) do