PERF: Speed up PostSerializer#reactions by avoiding nested loop tak…

…e 2 (#313)

This is a follow-up to 7467349.

Within the `object.post_actions.reject` loop, we are running through
another loop given by `object.post_actions_with_reaction_users&.find`.
When the `object.post_actions` and
`object.post_actions_with_reaction_users` array is large, we end up
spending alot of time executing the loops.

This commit resolves the problem by reducing the number of records we
loop in `object.post_actions_with_reaction_users` by prefiltering the
records by `post_action.id` to remove alot of unnecessary iterations in
the loop.

Local benchmarks for the method for 8000 items in `object.post_actions` records and 2000
items in `post_actions_with_reaction_users` shows the runtime of the
method decreasing from roughly 7seconds to about 5ms.

lib/discourse_reactions/topic_view_serializer_extension.rb

@@ -13,6 +13,11 @@ def self.load_post_action_reaction_users_for_posts(post_ids)
         "post_actions.post_id IN (#{DiscourseReactions::PostActionExtension.post_action_with_reaction_user_sql})",
         valid_reactions: DiscourseReactions::Reaction.reactions_counting_as_like,
       )
+      .reduce({}) do |hash, post_action|
+        hash[post_action.post_id] ||= {}
+        hash[post_action.post_id][post_action.id] = post_action
+        hash
+      end
   end
 
   def posts
@@ -29,8 +34,7 @@ def posts
         )
 
       posts.each do |post|
-        post.post_actions_with_reaction_users =
-          post_actions_with_reaction_users.select { |post_action| post_action.post_id == post.id }
+        post.post_actions_with_reaction_users = post_actions_with_reaction_users[post.id] || {}
       end
 
       object.instance_variable_set(:@posts, posts)

plugin.rb

@@ -112,14 +112,10 @@ class Engine < ::Rails::Engine
 
         # Also get rid of any PostAction records that match up to a ReactionUser
         # that is now the main_reaction_id and has historical data.
-        object
-          .post_actions_with_reaction_users
-          &.find do |pa|
-            pa.id == post_action.id &&
-              pa.reaction_user&.reaction&.reaction_value ==
-                DiscourseReactions::Reaction.main_reaction_id
-          end
-          .present?
+        object.post_actions_with_reaction_users[post_action.id]
+          &.reaction_user
+          &.reaction
+          &.reaction_value == DiscourseReactions::Reaction.main_reaction_id
       end
 
     # Likes will only be blank if there are only reactions where the reaction is in

spec/serializers/post_serializer_spec.rb

@@ -45,9 +45,11 @@
     reaction_user_1 && reaction_user_2 && reaction_user_3 && like
 
     post_1.post_actions_with_reaction_users =
-      DiscourseReactions::TopicViewSerializerExtension
-        .load_post_action_reaction_users_for_posts([post_1.id])
-        .select { |pa| pa.post_id == post_1.id }
+      DiscourseReactions::TopicViewSerializerExtension.load_post_action_reaction_users_for_posts(
+        [post_1.id],
+      )[
+        post_1.id
+      ]
   end
 
   it "renders custom reactions which should be sorted by count" do