Make sure users only vote once in the same poll

When skipping verification, we cannot apply the validation rule saying
the document number and document type must be unique, because they'll be
`nil` in many cases. So we were skipping the rule, but that makes it
possible for the same user to vote several times (for instance, once in
a booth and once via web).

So we're changing the scope of the uniqueness rule: instead of being
unique per document number, voters are unique per user. The reason we
made them unique per document number was that back in commit 900563e3
(when we added the rule), we hadn't added the relation between users and
poll voters yet.

app/models/poll/voter.rb

@@ -14,7 +14,8 @@ class Poll
     validates :booth_assignment_id, presence: true, if: ->(voter) { voter.origin == "booth" }
     validates :officer_assignment_id, presence: true, if: ->(voter) { voter.origin == "booth" }
 
-    validates :document_number, presence: true, uniqueness: { scope: [:poll_id, :document_type], message: :has_voted }, unless: :skip_user_verification?
+    validates :document_number, presence: true, unless: :skip_user_verification?
+    validates :user_id, uniqueness: { scope: [:poll_id], message: :has_voted }
     validates :origin, inclusion: { in: VALID_ORIGINS }
 
     before_validation :set_demographic_info, :set_document_info, :set_denormalized_booth_assignment_id

config/locales/en/activerecord.yml

@@ -508,6 +508,7 @@ en:
           attributes:
             document_number:
               not_in_census: "Document not in census"
+            user_id:
               has_voted: "User has already voted"
         legislation/process:
           attributes:

config/locales/es/activerecord.yml

@@ -510,6 +510,7 @@ es:
           attributes:
             document_number:
               not_in_census: "Este documento no aparece en el censo"
+            user_id:
               has_voted: "Este usuario ya ha votado"
         legislation/process:
           attributes:

spec/models/poll/voter_spec.rb

@@ -34,7 +34,7 @@ describe Poll::Voter do
       voter = build(:poll_voter, user: user, poll: poll)
 
       expect(voter).not_to be_valid
-      expect(voter.errors.messages[:document_number]).to eq(["User has already voted"])
+      expect(voter.errors.messages[:user_id]).to eq(["User has already voted"])
     end
 
     it "is not valid if the user has already voted in the same poll/booth" do
@@ -43,7 +43,7 @@ describe Poll::Voter do
       voter = build(:poll_voter, user: user, poll: poll, booth_assignment: booth_assignment)
 
       expect(voter).not_to be_valid
-      expect(voter.errors.messages[:document_number]).to eq(["User has already voted"])
+      expect(voter.errors.messages[:user_id]).to eq(["User has already voted"])
     end
 
     it "is not valid if the user has already voted in different booth in the same poll" do
@@ -52,7 +52,7 @@ describe Poll::Voter do
       voter = build(:poll_voter, :from_booth, user: user, poll: poll, booth: booth)
 
       expect(voter).not_to be_valid
-      expect(voter.errors.messages[:document_number]).to eq(["User has already voted"])
+      expect(voter.errors.messages[:user_id]).to eq(["User has already voted"])
     end
 
     it "is valid if the user has already voted in the same booth in different poll" do
@@ -69,7 +69,31 @@ describe Poll::Voter do
 
       voter = build(:poll_voter, poll: answer.question.poll, user: answer.author)
       expect(voter).not_to be_valid
-      expect(voter.errors.messages[:document_number]).to eq(["User has already voted"])
+      expect(voter.errors.messages[:user_id]).to eq(["User has already voted"])
+    end
+
+    context "Skip verification is enabled" do
+      before do
+        Setting["feature.user.skip_verification"] = true
+        user.update!(document_number: nil, document_type: nil)
+      end
+
+      it "is not valid if the user has already voted in the same poll" do
+        create(:poll_voter, user: user, poll: poll)
+
+        voter = build(:poll_voter, user: user, poll: poll)
+
+        expect(voter).not_to be_valid
+      end
+
+      it "is valid if other users have voted in the same poll" do
+        another_user = create(:user, :level_two, document_number: nil, document_type: nil)
+        create(:poll_voter, user: another_user, poll: poll)
+
+        voter = build(:poll_voter, user: user, poll: poll)
+
+        expect(voter).to be_valid
+      end
     end
 
     context "origin" do