FIX: Use unread post excerpt for topic-level bookmark excerpt (#14414)

In the user bookmark list, when we show the excerpt of the bookmark
(which is usually just the bookmarked post excerpt), we want to show
the first unread post's excerpt instead for for_topic bookmarks. This
is because when the user clicks on that bookmark link, they are taken
to the first unread post in the topic, not the OP, as per:

27699648ef

app/serializers/user_bookmark_serializer.rb

@@ -109,7 +109,24 @@ class UserBookmarkSerializer < ApplicationSerializer
   end
 
   def cooked
-    post.cooked
+    @cooked ||= \
+      if object.for_topic && last_read_post_number.present?
+        for_topic_cooked_post
+      else
+        post.cooked
+      end
+  end
+
+  def for_topic_cooked_post
+    post_number = [last_read_post_number + 1, highest_post_number].min
+    posts = Post.where(topic: topic, post_type: Post.types[:regular]).order(:post_number)
+    first_unread_cooked = posts.where("post_number >= ?", post_number).pluck_first(:cooked)
+
+    # if first_unread_cooked is blank this likely means that the last
+    # read post was either deleted or is a small action post.
+    # in this case we should just get the last regular post and
+    # use that for the cooked value so we have something to show
+    first_unread_cooked || posts.last.cooked
   end
 
   def slug

spec/serializers/user_bookmark_serializer_spec.rb

@@ -30,6 +30,7 @@ RSpec.describe UserBookmarkSerializer do
     expect(s.post_user_username).to eq(bookmark.post.user.username)
     expect(s.post_user_name).to eq(bookmark.post.user.name)
     expect(s.post_user_avatar_template).not_to eq(nil)
+    expect(s.excerpt).to eq(PrettyText.excerpt(post.cooked, 300, keep_emoji_images: true))
   end
 
   it "uses the correct highest_post_number column based on whether the user is staff" do
@@ -45,4 +46,39 @@ RSpec.describe UserBookmarkSerializer do
 
     expect(serializer.highest_post_number).to eq(4)
   end
+
+  context "for_topic bookmarks" do
+    before do
+      bookmark.update!(for_topic: true)
+    end
+
+    it "uses the last_read_post_number + 1 for the for_topic bookmarks excerpt" do
+      next_unread_post = Fabricate(:post_with_long_raw_content, topic: bookmark.topic)
+      Fabricate(:post_with_external_links, topic: bookmark.topic)
+      bookmark.reload
+      TopicUser.change(user.id, bookmark.topic.id, { last_read_post_number: post.post_number })
+      serializer = UserBookmarkSerializer.new(bookmark, scope: Guardian.new(user))
+      expect(serializer.excerpt).to eq(PrettyText.excerpt(next_unread_post.cooked, 300, keep_emoji_images: true))
+    end
+
+    it "does not use a small post for the last unread cooked post" do
+      small_action_post = Fabricate(:small_action, topic: bookmark.topic)
+      next_unread_post = Fabricate(:post_with_long_raw_content, topic: bookmark.topic)
+      Fabricate(:post_with_external_links, topic: bookmark.topic)
+      bookmark.reload
+      TopicUser.change(user.id, bookmark.topic.id, { last_read_post_number: post.post_number })
+      serializer = UserBookmarkSerializer.new(bookmark, scope: Guardian.new(user))
+      expect(serializer.excerpt).to eq(PrettyText.excerpt(next_unread_post.cooked, 300, keep_emoji_images: true))
+    end
+
+    it "handles the last read post in the topic being a small post by getting the last read regular post" do
+      last_regular_post = Fabricate(:post_with_long_raw_content, topic: bookmark.topic)
+      small_action_post = Fabricate(:small_action, topic: bookmark.topic)
+      bookmark.reload
+      TopicUser.change(user.id, bookmark.topic.id, { last_read_post_number: small_action_post.post_number })
+      serializer = UserBookmarkSerializer.new(bookmark, scope: Guardian.new(user))
+      expect(serializer.cooked).to eq(last_regular_post.cooked)
+      expect(serializer.excerpt).to eq(PrettyText.excerpt(last_regular_post.cooked, 300, keep_emoji_images: true))
+    end
+  end
 end