changes officer assignments to polls via new model/relation

app/models/poll/booth_assignment.rb

@@ -2,5 +2,8 @@ class Poll
   class BoothAssignment < ActiveRecord::Base
     belongs_to :booth
     belongs_to :poll
+
+    has_many :officer_assignments, class_name: "Poll::OfficerAssignment", dependent: :destroy
+    has_many :officers, through: :officer_assignments
   end
 end