[Fix #1513] Tweak autocorrection code for Rails/FindByOrAssignmentMemoization

Follow-up to rubocop/rails-style-guide#368. Closes #1513.

Author: koic

lib/rubocop/cop/rails/find_by_or_assignment_memoization.rb

@@ -20,11 +20,9 @@ module Rails
  #
  # # good
  # def current_user
- # if instance_variable_defined?(:@current_user)
- # @current_user
- # else
- # @current_user = User.find_by(id: session[:user_id])
- # end
+ # return @current_user if defined?(@current_user)
+ #
+ # @current_user = User.find_by(id: session[:user_id])
  # end
  class FindByOrAssignmentMemoization < Base
  extend AutoCorrector
@@ -49,11 +47,9 @@ def on_send(node)
  corrector.replace(
  assignment_node,
  <<~RUBY.rstrip
- if instance_variable_defined?(:#{varible_name})
- #{varible_name}
- else
- #{varible_name} = #{find_by.source}
- end
+ return #{varible_name} if defined?(#{varible_name})
+
+ #{varible_name} = #{find_by.source}
  RUBY
  )
  end

spec/rubocop/cop/rails/find_by_or_assignment_memoization_spec.rb

@@ -9,11 +9,9 @@
  RUBY
 
  expect_correction(<<~RUBY)
- if instance_variable_defined?(:@current_user)
- @current_user
- else
- @current_user = User.find_by(id: session[:user_id])
- end
+ return @current_user if defined?(@current_user)
+
+ @current_user = User.find_by(id: session[:user_id])
  RUBY
  end
  end