Specify dependent: destroy for UserBadges.

app/models/badge.rb

@@ -1,5 +1,6 @@
 class Badge < ActiveRecord::Base
   belongs_to :badge_type
+  has_many :user_badges, dependent: :destroy
 
   validates :name, presence: true, uniqueness: true
   validates :badge_type, presence: true

spec/models/badge.rb

@@ -4,6 +4,7 @@ require_dependency 'badge'
 describe Badge do
 
   it { should belong_to :badge_type }
+  it { should have_many(:user_badges).dependent(:destroy) }
 
   context 'validations' do
     before(:each) { Fabricate(:badge) }