comments: allow votes to be soft deleted.

Make it so that when a user removes their own vote, the vote is soft
deleted (the is_deleted flag is set) instead of hard deleted.

Changes:

* Add is_deleted flag to comment votes.
* Relax uniqueness constraint so you can have multiple deleted votes on
  the same comment. You can still only have one active vote on the comment.
* Add `soft_delete` method to Deletable concern.

test/unit/comment_vote_test.rb

@@ -10,11 +10,10 @@ class CommentVoteTest < ActiveSupport::TestCase
     context "during validation" do
       subject { build(:comment_vote, comment: as(@user) { create(:comment) }) }
 
-      should validate_uniqueness_of(:user_id).scoped_to(:comment_id).with_message("have already voted for this comment")
       should validate_inclusion_of(:score).in_array([-1, 1]).with_message("must be 1 or -1")
     end
 
-    should "not allow creating duplicate votes" do
+    should "not allow creating duplicate active votes" do
       v1 = create(:comment_vote, comment: @comment, user: @user)
       v2 = build(:comment_vote, comment: @comment, user: @user)
 
@@ -23,6 +22,16 @@ class CommentVoteTest < ActiveSupport::TestCase
       end
     end
 
+    should "allow creating duplicate deleted votes" do
+      v1 = create(:comment_vote, comment: @comment, user: @user)
+      v2 = create(:comment_vote, comment: @comment, user: @user, is_deleted: true)
+      v3 = create(:comment_vote, comment: @comment, user: @user, is_deleted: true)
+
+      assert_equal(true, v1.valid?)
+      assert_equal(true, v2.valid?)
+      assert_equal(true, v3.valid?)
+    end
+
     context "creating" do
       context "an upvote" do
         should "increment the comment's score" do
@@ -41,14 +50,15 @@ class CommentVoteTest < ActiveSupport::TestCase
       end
     end
 
-    context "destroying" do
+    context "soft deleting" do
       context "an upvote" do
         should "decrement the comment's score" do
           vote = create(:comment_vote, comment: @comment, score: 1)
           assert_equal(1, @comment.reload.score)
 
-          vote.destroy
+          vote.soft_delete(updater: vote.user)
           assert_equal(0, @comment.reload.score)
+          assert_equal(true, vote.is_deleted?)
         end
       end
 
@@ -57,8 +67,9 @@ class CommentVoteTest < ActiveSupport::TestCase
           vote = create(:comment_vote, comment: @comment, score: -1)
           assert_equal(-1, @comment.reload.score)
 
-          vote.destroy
+          vote.soft_delete(updater: vote.user)
           assert_equal(0, @comment.reload.score)
+          assert_equal(true, vote.is_deleted?)
         end
       end
     end