DRYup shared model validations on before_destroy - ruby-on-rails-3

Question

To prevent removal of related records, I am applying the before_destroy callbacks approach on each model
I defined several related-records validation methods in a module, so that they can be shared to different models' before_destroy callbacks:
class Teacher < ActiveRecord::Base
include RelatedModels
before_destroy :has_courses
...
end
class Level < ActiveRecord::Base
include RelatedModels
before_destroy :has_courses
...
end
module RelatedModels
def has_courses
if self.courses.any?
self.errors[:base] << "You cannot delete this while associated courses exists"
return false
end
end
def has_reports
if self.reports.any?
self.errors[:base] << "You cannot delete this while associated reports exists"
return false
end
end
def has_students
if self.students.any?
self.errors[:base] << "You cannot delete this while associated students exists"
return false
end
end
...
end
But it doesn't looks very DRY
Any idea how to do it in a single method?
Meta-programming it's not among my skills
Thanks in advance

Answer 1

You might want to try an observer class
These are depreciated in Rails 4 (you have to use the rails-observers gem), but seem to be still part of the Rails 3 core
Observer Classes
"Listen" to specified actions in your model, and provide extended functionality. As stated by their documentation, they are particularly good for de-cluttering your models, allowing you to combine a lot of functionality into one central set of functions
Here's what you might want to do (please bare in mind I have only used these in Rails 4):
class RelatedObserver < ActiveRecord::Observer
observe :teachers, :school
def before_destroy(record)
associations = %w(teachers schools reports)
associations.each do |association|
if record.send(association).any?
self.errors[:base] << "You cannot delete this while associated #{association.pluralize} exists"
return false
end
end
end
end
Here's a good resource for you