PERF: Remove N+1 query when generating posters summary.

lib/primary_group_lookup.rb

@@ -24,7 +24,9 @@ class PrimaryGroupLookup
       .select(:id, :primary_group_id)
 
     group_lookup = {}
-    group_ids = users_with_primary_group.map(&:primary_group_id).compact
+    group_ids = users_with_primary_group.map(&:primary_group_id)
+    group_ids.uniq!
+
     Group.where(id: group_ids).select(self.class.lookup_columns)
       .each { |g| group_lookup[g.id] = g }
 

lib/topic_query.rb

@@ -346,8 +346,13 @@ class TopicQuery
       end
 
       avatar_lookup = AvatarLookup.new(user_ids)
+      primary_group_lookup = PrimaryGroupLookup.new(user_ids)
+
       topics.each do |t|
-        t.posters = t.posters_summary(avatar_lookup: avatar_lookup)
+        t.posters = t.posters_summary(
+          avatar_lookup: avatar_lookup,
+          primary_group_lookup: primary_group_lookup
+        )
       end
     end