FEATURE: primary group class on avatars in topic list

lib/primary_group_lookup.rb

@@ -0,0 +1,37 @@
+class PrimaryGroupLookup
+  def initialize(user_ids=[])
+    @user_ids = user_ids.tap(&:compact!).tap(&:uniq!).tap(&:flatten!)
+  end
+
+  # Lookup primary group for a given user id
+  def [](user_id)
+    users[user_id]
+  end
+
+  private
+
+  def self.lookup_columns
+    @lookup_columns ||= %i{id name flair_url flair_bg_color flair_color}
+  end
+
+  def users
+    @users ||= user_lookup_hash
+  end
+
+  def user_lookup_hash
+    users_with_primary_group = User.where(id: @user_ids)
+        .where.not(primary_group_id: nil)
+        .select(:id, :primary_group_id)
+
+    group_lookup = {}
+    group_ids = users_with_primary_group.map(&:primary_group_id).compact
+    Group.where(id: group_ids).select(self.class.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