PERF: Combine avatar_lookup and primary_group_lookup into user_lookup (#10253)

These two classes were running very similar queries, which could be expensive on large topics

lib/user_lookup.rb

@@ -0,0 +1,59 @@
+# frozen_string_literal: true
+
+class UserLookup
+
+  def initialize(user_ids = [])
+    @user_ids = user_ids.tap(&:compact!).tap(&:uniq!).tap(&:flatten!)
+  end
+
+  # Lookup a user by id
+  def [](user_id)
+    users[user_id]
+  end
+
+  def primary_groups
+    @groups ||= group_lookup_hash
+  end
+
+  private
+
+  def self.lookup_columns
+    @user_lookup_columns ||= %i{id username name uploaded_avatar_id primary_group_id}
+  end
+
+  def self.group_lookup_columns
+    @group_lookup_columns ||= %i{id name flair_icon flair_upload_id flair_bg_color flair_color}
+  end
+
+  def users
+    @users ||= user_lookup_hash
+  end
+
+  def user_lookup_hash
+    hash = {}
+    User.where(id: @user_ids)
+      .select(self.class.lookup_columns)
+      .each { |user| hash[user.id] = user }
+    hash
+  end
+
+  def group_lookup_hash
+    users_with_primary_group = users.values.reject { |u| u.primary_group_id.nil? }
+
+    group_lookup = {}
+    group_ids = users_with_primary_group.map { |u| u.primary_group_id }
+    group_ids.uniq!
+
+    Group.includes(:flair_upload)
+      .where(id: group_ids)
+      .select(self.class.group_lookup_columns)
+      .each { |g| group_lookup[g.id] = g }
+
+    hash = {}
+    users_with_primary_group.each do |u|
+      hash[u.id] = group_lookup[u.primary_group_id]
+    end
+    hash
+  end
+
+end