PERF: avoid N+1 on topic view

Serializer is injecting information into cooked and reaching direct to
custom fields that were not preloaded

This amends it so basic post serializer can use the proper interface

That said we should probably follow this up so we don't reach for this
info on every post.

app/serializers/basic_post_serializer.rb

@@ -10,6 +10,8 @@ class BasicPostSerializer < ApplicationSerializer
              :cooked,
              :cooked_hidden
 
+  attr_accessor :topic_view
+
   def name
     object.user && object.user.name
   end
@@ -41,20 +43,27 @@ class BasicPostSerializer < ApplicationSerializer
       cooked = object.filter_quotes(@parent_post)
 
       if scope&.user
-        group = Group
-          .joins('JOIN group_users ON groups.id = group_users.group_id')
-          .find_by(
-            id: object.custom_fields['requested_group_id'].to_i,
-            group_users: { user_id: scope.user.id, owner: true }
-          )
 
-        if group
-          cooked << <<~EOF
-            <hr />
-            <a href="#{Discourse.base_uri}/g/#{group.name}/requests">
-              #{I18n.t('groups.request_membership_pm.handle')}
-            </a>
-          EOF
+        # PERF: this should not run on every post, only specific ones
+        # also, why is this in basic post serializer?
+        requested_group_id = post_custom_fields['requested_group_id'].to_i
+
+        if requested_group_id > 0
+          group = Group
+            .joins('JOIN group_users ON groups.id = group_users.group_id')
+            .find_by(
+              id: object.custom_fields['requested_group_id'].to_i,
+              group_users: { user_id: scope.user.id, owner: true }
+            )
+
+          if group
+            cooked << <<~EOF
+              <hr />
+              <a href="#{Discourse.base_uri}/g/#{group.name}/requests">
+                #{I18n.t('groups.request_membership_pm.handle')}
+              </a>
+            EOF
+          end
         end
       end
 
@@ -66,4 +75,12 @@ class BasicPostSerializer < ApplicationSerializer
     SiteSetting.enable_names?
   end
 
+  def post_custom_fields
+    @post_custom_fields ||= if @topic_view
+      (@topic_view.post_custom_fields || {})[object.id] || {}
+    else
+      object.custom_fields
+    end
+  end
+
 end

app/serializers/post_serializer.rb

@@ -4,7 +4,6 @@ class PostSerializer < BasicPostSerializer
 
   # To pass in additional information we might need
   INSTANCE_VARS ||= [
-    :topic_view,
     :parent_post,
     :add_raw,
     :add_title,
@@ -490,12 +489,4 @@ private
     @post_actions ||= (@topic_view&.all_post_actions || {})[object.id]
   end
 
-  def post_custom_fields
-    @post_custom_fields ||= if @topic_view
-      (@topic_view.post_custom_fields || {})[object.id] || {}
-    else
-      object.custom_fields
-    end
-  end
-
 end