MONGOID-5789 database_field_name given nil or empty string should raise UnknownAttribute exception (#5836)

* database_field_name given nil or empty string should raise UnknownAttribute exception

* fix spec syntax

* database_field_name return empty string instead of exception

* `field` might be an empty string, not merely nil

* fix specs to expect empty string instead of nil

This is okay, because the database_field_name method is a private API.
We can change the contract here without regard for who else might
be using it.

---------

Co-authored-by: Jamis Buck <jamis.buck@mongodb.com>

Author: danhealy

lib/mongoid/fields.rb

@@ -413,7 +413,8 @@ def traverse_association_tree(key, fields, associations, aliased_associations)
       #
       # @api private
       def database_field_name(name, relations, aliased_fields, aliased_associations)
-        return nil unless name.present?
+        return "" unless name.present?
+
         key = name.to_s
         segment, remaining = key.split('.', 2)
 

lib/mongoid/touchable.rb

@@ -78,7 +78,7 @@ def _gather_touch_updates(now, field = nil)
         field = database_field_name(field)
 
         write_attribute(:updated_at, now) if respond_to?("updated_at=")
-        write_attribute(field, now) if field
+        write_attribute(field, now) if field.present?
 
         touches = _extract_touches_from_atomic_sets(field) || {}
         touches.merge!(_parent._gather_touch_updates(now) || {}) if _touchable_parent?

spec/mongoid/attributes_spec.rb

@@ -289,6 +289,22 @@
         end
       end
 
+      context "when given nil" do
+
+        it "returns nil" do
+         expect(person[nil]).to be nil
+        end
+
+      end
+
+      context "when given an empty string" do
+
+        it "returns nil" do
+         expect(person[""]).to be nil
+        end
+
+      end
+
       context "when the field was not explicitly defined" do
 
         context "when excluding with only and the field was not excluded" do

spec/mongoid/fields_spec.rb

@@ -1871,12 +1871,12 @@ class DiscriminatorChild2 < DiscriminatorParent
 
     context 'given nil' do
       subject { Person.database_field_name(nil) }
-      it { is_expected.to eq nil }
+      it { is_expected.to eq '' }
     end
 
     context 'given an empty String' do
       subject { Person.database_field_name('') }
-      it { is_expected.to eq nil }
+      it { is_expected.to eq '' }
     end
 
     context 'given a String' do