Remove Poll::Voter record when there is no more user answers

Now we can remove answers we should provide a way of removing voting.

spec/models/poll/answer_spec.rb

@@ -106,4 +106,32 @@ describe Poll::Answer do
       expect(answer).not_to be_persisted
     end
   end
+
+  describe "#destroy_and_remove_voter_participation" do
+    let(:poll) { create(:poll) }
+    let(:question) { create(:poll_question, :yes_no, poll: poll) }
+
+    it "destroys voter record and answer when it was the only user's answer" do
+      answer = build(:poll_answer, question: question)
+      answer.save_and_record_voter_participation
+
+      expect { answer.destroy_and_remove_voter_participation }
+        .to change { Poll::Answer.count }.by(-1)
+        .and change { Poll::Voter.count }.by(-1)
+    end
+
+    it "destroys the answer but does not destroy the voter record when the user
+        has answered other poll questions" do
+      answer = build(:poll_answer, question: question)
+      answer.save_and_record_voter_participation
+      other_question = create(:poll_question, :yes_no, poll: poll)
+      other_answer = build(:poll_answer, question: other_question, author: answer.author)
+      other_answer.save_and_record_voter_participation
+
+      expect(other_answer).to be_persisted
+      expect { answer.destroy_and_remove_voter_participation }
+        .to change { Poll::Answer.count }.by(-1)
+        .and change { Poll::Voter.count }.by(0)
+    end
+  end
 end