Fix #4145: Unable to view deleted comments from post page.

Bug: if all the comments on a post were deleted then the deleted
comments wouldn't be visible to moderators.

This was because we assumed that if `last_commented_at` was nil it meant
that the post had no comments, but this was wrong. `last_commented_at`
only counts undeleted comments. It's reset to nil if all the commnets
have been deleted.

app/controllers/posts_controller.rb

@@ -23,7 +23,7 @@ class PostsController < ApplicationController
   def show
     @post = Post.find(params[:id])
 
-    @comments = @post.last_commented_at.present? ? @post.comments : Comment.none
+    @comments = @post.comments
     @comments = @comments.includes(:creator)
     @comments = @comments.includes(:votes) if CurrentUser.is_member?
     @comments = @comments.select { |c| c.visible_by?(CurrentUser.user) }

app/views/comments/partials/index/_list.html.erb

@@ -4,9 +4,9 @@
   <% end %>
 
   <div class="row notices">
-    <% if (post.last_commented_at.present? && post.comments.any? { |c| !c.visible_by?(CurrentUser.user) }) || (page == :comments && post.comments.size > 6) %>
+    <% if post.comments.any? { |c| !c.visible_by?(CurrentUser.user, show_deleted: true) } || (page == :comments && post.comments.size > 6) %>
       <span class="info" id="threshold-comments-notice-for-<%= post.id %>">
-        <%= link_to "Show all comments", comments_path(:post_id => post.id), :remote => true %>
+        <%= link_to "Show all comments", comments_path(post_id: post.id), id: "show-all-comments-link", remote: true %>
       </span>
     <% end %>
   </div>

test/functional/posts_controller_test.rb

@@ -143,6 +143,56 @@ class PostsControllerTest < ActionDispatch::IntegrationTest
         assert_response :success
       end
 
+      context "with only deleted comments" do
+        setup do
+          as(@user) { create(:comment, post: @post, is_deleted: true) }
+        end
+
+        should "not show deleted comments to regular members" do
+          get_auth post_path(@post), @user, params: { id: @post.id }
+
+          assert_response :success
+          assert_select "article.comment", 0
+          assert_select "a#show-all-comments-link", 0
+          assert_select "div.list-of-comments p", /There are no comments/
+        end
+
+        should "show deleted comments to moderators" do
+          mod = create(:mod_user)
+          get_auth post_path(@post), mod, params: { id: @post.id }
+
+          assert_response :success
+          assert_select "article.comment", 1
+        end
+      end
+
+      context "with only downvoted comments" do
+        should "not show thresholded comments" do
+          comment = as(@user) { create(:comment, post: @post, score: -10) }
+          get_auth post_path(@post), @user, params: { id: @post.id }
+
+          assert_response :success
+          assert_select "article.comment", 0
+          assert_select "a#show-all-comments-link", 1
+          assert_select "div.list-of-comments p", /There are no visible comments/
+        end
+      end
+
+      context "with a mix of comments" do
+        should "not show deleted or thresholded comments " do
+          as(@user) { create(:comment, post: @post, do_not_bump_post: true, body: "good") }
+          as(@user) { create(:comment, post: @post, do_not_bump_post: true, body: "bad", score: -10) }
+          as(@user) { create(:comment, post: @post, do_not_bump_post: true, body: "ugly", is_deleted: true) }
+
+          get_auth post_path(@post), @user, params: { id: @post.id }
+
+          assert_response :success
+          assert_select "article.comment", 1
+          assert_select "article.comment", /good/
+          assert_select "a#show-all-comments-link", 1
+        end
+      end
+
       context "when the recommend service is enabled" do
         setup do
           @post2 = create(:post)