Add unique index to poll participations on author_id and poll_id

Previously we had only a Rails validation which ensured poll participation
uniqueness but this adds uniqueness control to the database level, so that
uniqueness is guaranteed even when changing data with avoiding Rails
validations.

closes #7798

Changelog.md

 # 0.7.6.0
 
 ## Refactor
+* Add unique index to poll participations on `poll_id` and `author_id` [#7798](https://github.com/diaspora/diaspora/pull/7798)
 
 ## Bug fixes
 

db/migrate/20180425125409_add_poll_participations_unique_index_on_author_and_poll.rb

+# frozen_string_literal: true
+
+class AddPollParticipationsUniqueIndexOnAuthorAndPoll < ActiveRecord::Migration[5.1]
+  def change
+    reversible do |change|
+      change.up do
+        duplicate_query = "WHERE a1.poll_id = a2.poll_id AND a1.author_id = a2.author_id AND a1.id > a2.id"
+        if AppConfig.postgres?
+          execute("DELETE FROM poll_participations AS a1 USING poll_participations AS a2 #{duplicate_query}")
+        else
+          execute("DELETE a1 FROM poll_participations a1, poll_participations a2 #{duplicate_query}")
+        end
+      end
+    end
+
+    add_index :poll_participations, %i[poll_id author_id], unique: true
+    remove_index :poll_participations, :poll_id
+  end
+end

spec/models/poll_participation_spec.rb

@@ -24,6 +24,16 @@ describe PollParticipation, type: :model do
         bob.participate_in_poll!(status, poll.poll_answers.first)
       }.to_not raise_error
     end
+
+    it "has unique DB index for author-person" do
+      pp = FactoryGirl.create(:poll_participation)
+      pp2 = FactoryGirl.create(:poll_participation, author: pp.author)
+      expect {
+        # rubocop:disable Rails/SkipsModelValidations
+        pp2.update_attribute(:poll_id, pp.poll_id)
+        # rubocop:enable Rails/SkipsModelValidations
+      }.to raise_error ActiveRecord::RecordNotUnique
+    end
   end
 
   it_behaves_like "it is relayable" do