MONGOID-5208 fix error on reloading nil embedded document

lib/mongoid/reloadable.rb

@@ -22,7 +22,7 @@ def reload
       end
 
       reloaded = _reload
-      if Mongoid.raise_not_found_error && reloaded.empty?
+      if Mongoid.raise_not_found_error && (reloaded.nil? || reloaded.empty?)
         raise Errors::DocumentNotFound.new(self.class, _id, _id)
       end
       @attributes = reloaded
@@ -78,7 +78,8 @@ def reload_embedded_document
     #
     # @param [ Hash ] attributes The document in the db.
     #
-    # @return [ Hash ] The document's extracted attributes.
+    # @return [ Hash | nil ] The document's extracted attributes or nil if the
+    #   document doesn't exist.
     def extract_embedded_attributes(attributes)
       atomic_position.split(".").inject(attributes) do |attrs, part|
         attrs = attrs[part =~ /\d/ ? part.to_i : part]

spec/mongoid/reloadable_spec.rb

@@ -311,6 +311,31 @@
       end
     end
 
+    context "when embedded document is nil" do
+
+      let(:palette) do
+        Palette.new
+      end
+
+      let(:canvas) do
+        Canvas.create!(palette: palette)
+      end
+
+      before do
+        canvas.palette = nil
+      end
+
+      let(:reload) do
+        palette.reload
+      end
+
+      it "raises a document not found error" do
+        expect do
+          reload
+        end.to raise_error(Mongoid::Errors::DocumentNotFound, /Document\(s\) not found for class Palette with id\(s\)/)
+      end
+    end
+
     context "with relational associations" do
 
       let(:person) do