Add missing validation rule to poll answers

We were checking we didn't have more votes than allowed in the case of
questions with multiple answers, but we weren't checking it in the case
of questions with a single answer. This made it possible to create more
than one answer to the same question. This could happen because the
method `find_or_initialize_user_answer` might initialize two answers in
different threads, due to a race condition.

app/models/poll/answer.rb

@@ -36,7 +36,7 @@ class Poll::Answer < ApplicationRecord
   private
 
     def max_votes
-      return if !question || !author || question&.unique? || persisted?
+      return if !question || !author || persisted?
 
       author.lock!
 

app/models/poll/question.rb

@@ -28,7 +28,7 @@ class Poll::Question < ApplicationRecord
   accepts_nested_attributes_for :question_options, reject_if: :all_blank, allow_destroy: true
   accepts_nested_attributes_for :votation_type
 
-  delegate :max_votes, :multiple?, :vote_type, to: :votation_type, allow_nil: true
+  delegate :multiple?, :vote_type, to: :votation_type, allow_nil: true
 
   scope :by_poll_id,    ->(poll_id) { where(poll_id: poll_id) }
 
@@ -90,6 +90,14 @@ class Poll::Question < ApplicationRecord
     votation_type.nil? || votation_type.unique?
   end
 
+  def max_votes
+    if multiple?
+      votation_type.max_votes
+    else
+      1
+    end
+  end
+
   def find_or_initialize_user_answer(user, title)
     answer = answers.find_or_initialize_by(find_by_attributes(user, title))
     answer.answer = title

spec/models/poll/answer_spec.rb

@@ -30,6 +30,17 @@ describe Poll::Answer do
       expect(answer).not_to be_valid
     end
 
+    it "is not valid if there's already an answer to that question" do
+      author = create(:user)
+      question = create(:poll_question, :yes_no)
+
+      create(:poll_answer, author: author, question: question)
+
+      answer = build(:poll_answer, author: author, question: question)
+
+      expect(answer).not_to be_valid
+    end
+
     it "is not valid when user already reached multiple answers question max votes" do
       author = create(:user)
       question = create(:poll_question_multiple, :abc, max_votes: 2)
@@ -40,20 +51,6 @@ describe Poll::Answer do
       expect(answer).not_to be_valid
     end
 
-    it "validates max votes when creating answers at the same time", :race_condition do
-      author = create(:user, :level_two)
-      question = create(:poll_question_multiple, :abc, max_votes: 2)
-      create(:poll_answer, question: question, answer: "Answer A", author: author)
-      answer = build(:poll_answer, question: question, answer: "Answer B", author: author)
-      other_answer = build(:poll_answer, question: question, answer: "Answer C", author: author)
-
-      [answer, other_answer].map do |a|
-        Thread.new { a.save }
-      end.each(&:join)
-
-      expect(Poll::Answer.count).to be 2
-    end
-
     it "is valid for answers included in the Poll::Question's question_options list" do
       question = create(:poll_question)
       create(:poll_question_option, title: "One", question: question)
@@ -66,6 +63,36 @@ describe Poll::Answer do
 
       expect(build(:poll_answer, question: question, answer: "Four")).not_to be_valid
     end
+
+    context "creating answers at the same time", :race_condition do
+      it "validates max votes on single-answer questions" do
+        author = create(:user)
+        question = create(:poll_question, :yes_no)
+
+        answer = build(:poll_answer, author: author, question: question, answer: "Yes")
+        other_answer = build(:poll_answer, author: author, question: question, answer: "No")
+
+        [answer, other_answer].map do |poll_answer|
+          Thread.new { poll_answer.save }
+        end.each(&:join)
+
+        expect(Poll::Answer.count).to be 1
+      end
+
+      it "validates max votes on multiple-answer questions" do
+        author = create(:user, :level_two)
+        question = create(:poll_question_multiple, :abc, max_votes: 2)
+        create(:poll_answer, question: question, answer: "Answer A", author: author)
+        answer = build(:poll_answer, question: question, answer: "Answer B", author: author)
+        other_answer = build(:poll_answer, question: question, answer: "Answer C", author: author)
+
+        [answer, other_answer].map do |poll_answer|
+          Thread.new { poll_answer.save }
+        end.each(&:join)
+
+        expect(Poll::Answer.count).to be 2
+      end
+    end
   end
 
   describe "#save_and_record_voter_participation" do
@@ -92,14 +119,14 @@ describe Poll::Answer do
 
       expect(poll.reload.voters.size).to eq(1)
 
-      answer = create(:poll_answer, question: question, author: author, answer: "No")
+      updated_answer = answer.question.find_or_initialize_user_answer(answer.author, "No")
-      answer.save_and_record_voter_participation
+      updated_answer.save_and_record_voter_participation
 
       expect(poll.reload.voters.size).to eq(1)
 
       voter = poll.voters.first
-      expect(voter.document_number).to eq(answer.author.document_number)
+      expect(voter.document_number).to eq(updated_answer.author.document_number)
-      expect(voter.poll_id).to eq(answer.poll.id)
+      expect(voter.poll_id).to eq(updated_answer.poll.id)
     end
 
     it "does not save the answer if the voter is invalid" do
@@ -112,6 +139,22 @@ describe Poll::Answer do
 
       expect(answer).not_to be_persisted
     end
+
+    it "does not create two voters when creating two answers at the same time", :race_condition do
+      answer = build(:poll_answer, question: question, author: author, answer: "Yes")
+      other_answer = build(:poll_answer, question: question, author: author, answer: "No")
+
+      [answer, other_answer].map do |poll_answer|
+        Thread.new do
+          begin
+            poll_answer.save_and_record_voter_participation
+          rescue ActiveRecord::RecordInvalid
+          end
+        end
+      end.each(&:join)
+
+      expect(Poll::Voter.count).to be 1
+    end
   end
 
   describe "#destroy_and_remove_voter_participation" do