validates user has not already voted

app/models/poll.rb

@@ -1,2 +1,4 @@
 class Poll < ActiveRecord::Base
-end
+  has_many :booths
+  has_many :voters, through: :booths, class_name: "Poll::Voter"
+end

app/models/poll/booth.rb

@@ -0,0 +1,6 @@
+class Poll
+  class Booth < ActiveRecord::Base
+    belongs_to :poll
+    has_many :voters
+  end
+end

app/models/poll/voter.rb

@@ -1,15 +1,26 @@
 class Poll
   class Voter < ActiveRecord::Base
     belongs_to :booth
+    delegate :poll, to: :booth
 
     validate :in_census
+    validate :has_not_voted
 
     def in_census
       errors.add(:document_number, :not_in_census) unless census_api_response.valid?
     end
 
+    def has_not_voted
+      errors.add(:document_number, :has_voted) if has_voted?
+    end
+
     def census_api_response
       CensusApi.new.call(document_type, document_number)
     end
+
+    def has_voted?
+      poll.voters.where(document_number: document_number, document_type: document_type).present?
+    end
+
   end
 end