Add before_destroy :cancel to precent role destroy.Change Conference-Role association to have dependent: :destroy_all

app/models/role.rb

@@ -1,10 +1,17 @@
 class Role < ActiveRecord::Base
   belongs_to :resource, polymorphic: true
   has_and_belongs_to_many :users, join_table: :users_roles
-
+  before_destroy :cancel
   scopify
 
   validates :name, presence: true
 
   validates :name, uniqueness: { scope: :resource }
+
+  private
+
+  # Needed to ensure that removing all user from role doesn't remove role.
+  def cancel
+    false
+  end
 end