Question

We have a very simple group and role model

class Group < ActiveRecord::Base
  has_many :roles, :dependent => :destroy
end


class Role < ActiveRecord::Base
  belongs_to :group
  after_destroy :ensure_last_role

  private
  def ensure_last_role
    raise "Can't delete last role" if group.roles.count.zero?
  end  
end

The after_destroy works perfectly and doesn’t allow the last role to be destroyed by accident. BUT this also prevents the group from being destroyed when :dependent => :destroy tries to destroy all roles. Is it possible to not call ensure_last_role when a group gets destroyed or is there a better way to approach this?

Answer 1

After playing around with this a little while, I settled on a solution that doesn’t use the Rails built in dependent: :destroy for the relation, as I couldn’t get the callbacks to fire in the right order. There were also some issues with caching, so each time you try to destroy a Role it will make a small SQL query to see if its group still exists in the database:

> role.destroy
  Group Exists (0.2ms)  SELECT 1 FROM "groups" WHERE "groups"."id" = 18 LIMIT 1

At any rate, here’s the full model code:

class Group < ActiveRecord::Base
  has_many :roles
  after_destroy :destroy_roles

  def destroy_roles
    roles.each { |r| r.destroy }
  end
end

class Role < ActiveRecord::Base
  belongs_to :group
  before_destroy :ensure_last_role, if: "Group.exists?(group_id)"

  private

  def ensure_last_role
    raise "Can't delete last role" if group.roles.count == 1
  end
end