FEATURE: add a Top Categories section to the user summary page, showing the categories in which a user has the most activity

app/models/user_summary.rb

@@ -155,6 +155,56 @@ class UserSummary
     @user.recent_time_read
   end
 
+  class CategoryWithCounts < OpenStruct
+    include ActiveModel::SerializerSupport
+    KEYS = [:id, :name, :color, :text_color, :slug, :read_restricted, :parent_category_id]
+  end
+
+  def top_categories
+    post_count_query = Post
+      .joins(:topic)
+      .includes(:topic)
+      .secured(@guardian)
+      .merge(Topic.listable_topics.visible.secured(@guardian))
+      .where(user: @user)
+      .group('topics.category_id')
+      .order('COUNT(*) DESC')
+
+    top_categories = {}
+
+    Category.where(id: post_count_query.limit(MAX_SUMMARY_RESULTS).pluck('category_id'))
+      .pluck(:id, :name, :color, :text_color, :slug, :read_restricted, :parent_category_id)
+      .each do |c|
+        top_categories[c[0].to_i] = CategoryWithCounts.new(
+          Hash[CategoryWithCounts::KEYS.zip(c)].merge(
+            topic_count: 0,
+            post_count: 0
+          )
+        )
+      end
+
+    post_count_query.where('post_number > 1')
+      .where('topics.category_id in (?)', top_categories.keys)
+      .pluck('category_id, COUNT(*)')
+      .each do |r|
+        top_categories[r[0].to_i].post_count = r[1]
+      end
+
+    Topic.listable_topics.visible.secured(@guardian)
+      .where('topics.category_id in (?)', top_categories.keys)
+      .where(user: @user)
+      .group('topics.category_id')
+      .order('COUNT(*) DESC')
+      .pluck('category_id, COUNT(*)')
+      .each do |r|
+        top_categories[r[0].to_i].topic_count = r[1]
+      end
+
+    top_categories.values.sort_by do |r|
+      -(r[:post_count] + r[:topic_count])
+    end
+  end
+
   delegate :likes_given,
            :likes_received,
            :days_visited,