Fix MONGOID-5136 .touch with custom field is broken for embedded documents

lib/mongoid/atomic.rb

@@ -385,11 +385,35 @@ def touch_atomic_updates(field = nil)
       updates = atomic_updates
       return {} unless atomic_updates.key?("$set")
       touches = {}
+      wanted_keys = %w(updated_at u_at)
+      # TODO this permits field to be passed as an empty string in which case
+      # it is ignored, get rid of this behavior.
+      if field.present?
+        wanted_keys << field.to_s
+      end
       updates["$set"].each_pair do |key, value|
-        key_regex = /updated_at|u_at#{"|" + field if field.present?}/
-        touches.merge!({ key => value }) if key =~ key_regex
+        if wanted_keys.include?(key.split('.').last)
+          touches.update(key => value)
+        end
       end
       { "$set" => touches }
     end
+
+    # Returns the $set atomic updates affecting the specified field.
+    #
+    # @param [ String ] field The field name.
+    #
+    # @api private
+    def set_field_atomic_updates(field)
+      updates = atomic_updates
+      return {} unless atomic_updates.key?("$set")
+      sets = {}
+      updates["$set"].each_pair do |key, value|
+        if key.split('.').last == field
+          sets.update(key => value)
+        end
+      end
+      { "$set" => sets }
+    end
   end
 end

lib/mongoid/touchable.rb

@@ -41,6 +41,16 @@ def touch(field = nil)
           # _association.inverse_association.options but inverse_association
           # seems to not always/ever be set here. See MONGOID-5014.
           _parent.touch
+
+          if field
+            # If we are told to also touch a field, perform a separate write
+            # for that field. See MONGOID-5136.
+            # In theory we should combine the writes, which would require
+            # passing the fields to be updated to the parents - MONGOID-5142.
+            sets = set_field_atomic_updates(field)
+            selector = atomic_selector
+            _root.collection.find(selector).update_one(positionally(selector, sets), session: _session)
+          end
         else
           # If the current document is not embedded, it is composition root
           # and we need to persist the write here.

spec/mongoid/touchable_spec.rb

@@ -132,6 +132,24 @@
         include_examples 'updates the child'
         include_examples 'updates the parent when :touch is true'
         include_examples 'updates the parent when :touch is not set'
+
+        context 'when also updating an additional field' do
+          it 'persists the update to the additional field' do
+            entrance
+            update_time
+            entrance.touch(:last_used_at)
+
+            entrance.reload
+            building.reload
+
+            # This is the assertion we want.
+            entrance.last_used_at.should == update_time
+
+            # Check other timestamps for good measure.
+            entrance.updated_at.should == update_time
+            building.updated_at.should == update_time
+          end
+        end
       end
 
       context "when the document is referenced" do

spec/mongoid/touchable_spec_models.rb

@@ -16,6 +16,8 @@ class Entrance
       include Mongoid::Timestamps
 
       embedded_in :building
+
+      field :last_used_at, type: Time
     end
 
     class Floor