comments: add uniqueness constraint on votes.

db/migrate/20210123112752_add_unique_user_id_index_to_comment_votes.rb

@@ -0,0 +1,5 @@
+class AddUniqueUserIdIndexToCommentVotes < ActiveRecord::Migration[6.1]
+  def change
+    add_index :comment_votes, [:user_id, :comment_id], unique: true
+  end
+end

db/structure.sql

@@ -5008,10 +5008,11 @@ CREATE INDEX index_comment_votes_on_user_id ON public.comment_votes USING btree
 
 
 --
--- Name: index_comments_on_created_at; Type: INDEX; Schema: public; Owner: -
+-- Name: index_comment_votes_on_user_id_and_comment_id; Type: INDEX; Schema: public; Owner: -
 --
 
-CREATE INDEX index_comments_on_created_at ON public.comments USING btree (created_at);
+CREATE UNIQUE INDEX index_comment_votes_on_user_id_and_comment_id ON public.comment_votes USING btree (user_id, comment_id);
+
 
 --
 -- Name: index_comments_on_body_index; Type: INDEX; Schema: public; Owner: -
@@ -5020,6 +5021,13 @@ CREATE INDEX index_comments_on_created_at ON public.comments USING btree (create
 CREATE INDEX index_comments_on_body_index ON public.comments USING gin (body_index);
 
 
+--
+-- Name: index_comments_on_created_at; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_comments_on_created_at ON public.comments USING btree (created_at);
+
+
 --
 -- Name: index_comments_on_creator_id_and_post_id; Type: INDEX; Schema: public; Owner: -
 --
@@ -7883,6 +7891,7 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20210108030724'),
 ('20210110015410'),
 ('20210110090656'),
-('20210115015308');
+('20210115015308'),
+('20210123112752');
 
 

test/unit/comment_vote_test.rb

@@ -14,6 +14,15 @@ class CommentVoteTest < ActiveSupport::TestCase
       should validate_inclusion_of(:score).in_array([-1, 1]).with_message("must be 1 or -1")
     end
 
+    should "not allow creating duplicate votes" do
+      v1 = create(:comment_vote, comment: @comment, user: @user)
+      v2 = build(:comment_vote, comment: @comment, user: @user)
+
+      assert_raise(ActiveRecord::RecordNotUnique) do
+        v2.save(validate: false)
+      end
+    end
+
     context "creating" do
       context "an upvote" do
         should "increment the comment's score" do