Add auto-correction for Rails/HasManyOrHasOneDependent to be the Rails default (nil)

[jdufresne]

Is your feature request related to a problem? Please describe.

I have a large legacy code base. When adopting rubocop-rails defaults, the Rails/HasManyOrHasOneDependent presents many offenses that require manual code updates. As this code base already uses the default dependent implicitly, it seems reasonable that RuboCop could auto-correct the code to continue using it by specifying the argument explicitly.

The documentation for has_many / dependent is at:

https://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_many

Controls what happens to the associated objects when their owner is destroyed. ...

• nil do nothing (default).

Describe the solution you'd like

Given:

class User < ActiveRecord::Base
  has_many :comments
end

After running rubocup -A with Rails/HasManyOrHasOneDependent enabled:

class User < ActiveRecord::Base
  has_many :comments, dependent: nil
end

Describe alternatives you've considered

• Disabling this cop as manually correcting it is too much of a burden
• Manually correcting the offeneses