[Fix rubocop#6888] Fix an error for Rails/ActiveRecordOverride

Fixes rubocop#6888.

This PR fixes the following error for `Rails/ActiveRecordOverride`
when there is no `parent_class` to match against.

```ruby
% cat app/models/user.rb
class User
  def initialize; end

  def create; end
end
```

```console
% rubocop app/models/user.rb --only Rails/ActiveRecordOverride -d
For /Users/dannyiachini/github/jpw/rubocop_ex: configuration from /Users/dannyiachini/github/jpw/rubocop_ex/.rubocop.yml
Default configuration from /Users/dannyiachini/.gem/ruby/2.5.3/gems/rubocop-0.67.2/config/default.yml
Inspecting 1 file
Scanning /Users/dannyiachini/github/jpw/rubocop_ex/app/models/user.rb
An error occurred while Rails/ActiveRecordOverride cop was inspecting /Users/dannyiachini/github/jpw/rubocop_ex/app/models/user.rb:6:2.
undefined method `const_name' for nil:NilClass
/Users/dannyiachini/.gem/ruby/2.5.3/gems/rubocop-0.67.2/lib/rubocop/cop/rails/active_record_override.rb:42:in `on_def'

(snip)

1 file inspected, no offenses detected

1 error occurred:
An error occurred while Rails/ActiveRecordOverride cop was inspecting /Users/dannyiachini/github/jpw/rubocop_ex/app/models/user.rb:6:2.
Errors are usually caused by RuboCop bugs.
Please, report your problems to RuboCop's issue tracker.
https://github.com/rubocop-hq/rubocop/issues

Mention the following information in the issue report:
0.67.2 (using Parser 2.6.2.0, running on ruby 2.5.3 x86_64-darwin17)
```

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Bug fixes
 
 * [#6914](https://github.com/rubocop-hq/rubocop/issues/6914): [Fix #6914] Fix an error for `Rails/RedundantAllowNil` when with interpolations. ([@Blue-Pix][])
+* [#6888](https://github.com/rubocop-hq/rubocop/issues/6888): Fix an error for `Rails/ActiveRecordOverride ` when no `parent_class` present. ([@diachini][])
 
 ### Changes
 
@@ -3937,3 +3938,4 @@
 [@XrXr]: https://github.com/XrXr
 [@thomthom]: https://github.com/thomthom
 [@Blue-Pix]: https://github.com/Blue-Pix
+[@diachini]: https://github.com/diachini

lib/rubocop/cop/rails/active_record_override.rb

@@ -29,17 +29,15 @@ class ActiveRecordOverride < Cop
           'Use %<prefer>s callbacks instead of overriding the Active Record ' \
           'method `%<bad>s`.'.freeze
         BAD_METHODS = %i[create destroy save update].freeze
+        ACTIVE_RECORD_CLASSES = %w[ApplicationRecord ActiveModel::Base
+                                   ActiveRecord::Base].freeze
 
         def on_def(node)
           method_name = node.method_name
           return unless BAD_METHODS.include?(node.method_name)
 
-          parent_parts = node.parent.node_parts
-          parent_class = parent_parts.take_while do |part|
-            !part.nil? && part.const_type?
-          end.last
-          return unless %w[ApplicationRecord ActiveModel::Base]
-                        .include?(parent_class.const_name)
+          parent_class_name = find_parent_class_name(node)
+          return unless active_model?(parent_class_name)
 
           return unless node.descendants.any?(&:zsuper_type?)
 
@@ -48,6 +46,10 @@ def on_def(node)
 
         private
 
+        def active_model?(parent_class_name)
+          ACTIVE_RECORD_CLASSES.include?(parent_class_name)
+        end
+
         def callback_names(method_name)
           names = %w[before_ around_ after_].map do |prefix|
             "`#{prefix}#{method_name}`"
@@ -61,6 +63,20 @@ def callback_names(method_name)
         def message(method_name)
           format(MSG, prefer: callback_names(method_name), bad: method_name)
         end
+
+        def find_parent_class_name(node)
+          return nil unless node
+
+          if node.class_type?
+            parent_class_name = node.node_parts[1]
+
+            return nil if parent_class_name.nil?
+
+            return parent_class_name.source
+          end
+
+          find_parent_class_name(node.parent)
+        end
       end
     end
   end

spec/rubocop/cop/rails/active_record_override_spec.rb

@@ -41,6 +41,8 @@ def save
   it 'registers an offense when overriding update' do
     expect_offense(<<-RUBY.strip_indent)
       class X < ActiveModel::Base
+        module_function
+
         def update
         ^^^^^^^^^^ Use `before_update`, `around_update`, or `after_update` callbacks instead of overriding the Active Record method `update`.
           super
@@ -84,4 +86,16 @@ def save
       RUBY
     end
   end
+
+  context 'when class has no parent specified' do
+    it 'registers no offense when overriding save' do
+      expect_no_offenses(<<-RUBY.strip_indent)
+        class X
+          def initialize; end
+
+          def save; end
+        end
+      RUBY
+    end
+  end
 end