How to forbid deletion if association present - ruby-on-rails-3

Question

I have a many to many relationship between two models as follows:
#users.rb
has_many :users_to_roles
has_many :roles, through: :users_to_roles
#users_to_roles.rb
belongs_to :user
belongs_to :role
#roles.rb
has_many :users_to_roles
has_many :users, through: :users_to_roles
I want to disable the deletion of roles if there are users who are "in this role". Here I have found two options who should do the work:
:restrict_with_exception causes an exception to be raised if there are
any associated records :restrict_with_error causes an error to be
added to the owner if there are any associated objects
but there is no example with the syntax of this and how it should work.
Could you help to make this valid:
#roles.rb
has_many :users_to_roles
has_many :users, through: :users_to_roles, dependent: restrict_with_exception

Answer 1

Such operations can be easily do using Callbacks. In my case, I have added the following method in my model:
# callbacks
before_destroy :check_for_users_in_this_role
def check_for_users_in_this_role
status = true
if self.security_users.count > 0
self.errors[:deletion_status] = 'Cannot delete security role with active users in it.'
status = false
else
self.errors[:deletion_status] = 'OK.'
end
status
end

Answer 2

Alternatively, you can rescue the exception in your controller. In this example, a contact may own interest, i.e.
class Interest < ActiveRecord::Base
belongs_to :contact
end
class Contact < ActiveRecord::Base
has_many :interests, :dependent => :restrict
end
Then in the controller:
def destroy
#contact = Contact.find(params[:id])
begin
#contact.destroy
rescue
flash[:msg] = "Can't delete - owns interest"
end
respond_to do |format|
format.html { redirect_to(:back) }
format.xml { head :ok }
end
end
The flash message will be displayed in the calling page.

Answer 3

The correct rails way is to do the following:
users.rb:
has_many :users_to_roles, dependant: :destroy # don't keep the join table entry if the user is gone
has_many :roles, through: :users_to_roles
Make sure that your join does not have redundant entries (in which either column is null or orphaned).
users_to_roles.rb:
belongs_to :user
belongs_to :role
# add validations presence of both user and role
# in both model and database.
Bonus, from rails 4.2 you can add forigen_key: true in your migration for referential integrity
Now in your role (I am assuming you name your models singularly and made a typo in the question), you add this:
role.rb:
has_many :users_to_roles, dependant: :restrict_with_error
has_many :users, through: :users_to_roles

Answer 4

I made it with my classes like this:
app/models/guest_chat_token.rb
class GuestChatToken < ApplicationRecord
has_many :chat_messages, as: :sendable, dependent: :restrict_with_exception
end
app/controllers/admin/application_controller.rb
class Admin::ApplicationController < ApplicationController
....
rescue_from ActiveRecord::DeleteRestrictionError do |exception|
redirect_to :back, notice:
"Be aware: #{exception.message}."
end
end