Rails/HasManyOrHasOneDependent should not be an offence when the model is readonly

[nunosilva800]

When you a have a readonly model (for example a model backed by a database view):

class NiceThing < ActiveRecord::Base 
  def readonly?
     true
  end
end

and you define an has_many relation such as: has_many :foos, foreign_key: :bar_id, then it does not make sense to enforce a dependent option.

Maybe this cop should be ignored when the class is readonly?, or at least be configurable to ignore those.

Do you think this is worthwhile?
I can try to implement this, but I actually don't really now if there are good determinist ways to detect if the class is readonly?, maybe just checking that the method is defined and that it evals to true? Sounds complicated, and dangerous...