UX: Introduce automatic 'categories topics' setting (#8804)

When 'categories topics' setting is set to 0, the system will
automatically try to find a value to keep the two columns (categories
and topics) symmetrical.

The value is computed as 1.5x the number of top level categories and at
least 5 topics will always be returned.

app/controllers/categories_controller.rb

@@ -8,6 +8,9 @@ class CategoriesController < ApplicationController
   before_action :initialize_staff_action_logger, only: [:create, :update, :destroy]
   skip_before_action :check_xhr, only: [:index, :categories_and_latest, :categories_and_top, :redirect]
 
+  SYMMETRICAL_CATEGORIES_TO_TOPICS_FACTOR = 1.5
+  MIN_CATEGORIES_TOPICS = 5
+
   def redirect
     return if handle_permalink("/category/#{params[:path]}")
     redirect_to path("/c/#{params[:path]}")
@@ -46,7 +49,7 @@ class CategoriesController < ApplicationController
 
         style = SiteSetting.desktop_category_page_style
         topic_options = {
-          per_page: SiteSetting.categories_topics,
+          per_page: CategoriesController.topics_per_page,
           no_definitions: true
         }
 
@@ -226,6 +229,15 @@ class CategoriesController < ApplicationController
   end
 
   private
+
+  def self.topics_per_page
+    return SiteSetting.categories_topics if SiteSetting.categories_topics > 0
+
+    count = Category.where(parent_category: nil).count
+    count = (SYMMETRICAL_CATEGORIES_TO_TOPICS_FACTOR * count).to_i
+    count > MIN_CATEGORIES_TOPICS ? count : MIN_CATEGORIES_TOPICS
+  end
+
   def categories_and_topics(topics_filter)
     discourse_expires_in 1.minute
 
@@ -236,7 +248,7 @@ class CategoriesController < ApplicationController
     }
 
     topic_options = {
-      per_page: SiteSetting.categories_topics,
+      per_page: CategoriesController.topics_per_page,
       no_definitions: true
     }
 

config/locales/server.en.yml

@@ -1686,7 +1686,7 @@ en:
     alert_admins_if_errors_per_minute: "Number of errors per minute in order to trigger an admin alert. A value of 0 disables this feature. NOTE: requires restart."
     alert_admins_if_errors_per_hour: "Number of errors per hour in order to trigger an admin alert. A value of 0 disables this feature. NOTE: requires restart."
 
-    categories_topics: "Number of topics to show in /categories page."
+    categories_topics: "Number of topics to show in /categories page. If set to 0, it will automatically try to find a value to keep the two columns symmetrical (categories and topics)."
     suggested_topics: "Number of suggested topics shown at the bottom of a topic."
     limit_suggested_to_category: "Only show topics from the current category in suggested topics."
     suggested_topics_max_days_old: "Suggested topics should not be more than n days old."

config/site_settings.yml

@@ -135,8 +135,7 @@ basic:
     default: false
   categories_topics:
     default: 20
-    min: 5
+    validator: "CategoriesTopicsValidator"
-    max: 2000
   suggested_topics:
     client: true
     default: 5

lib/validators/categories_topics_validator.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+class CategoriesTopicsValidator
+  def initialize(opts = {})
+    @opts = opts
+  end
+
+  def valid_value?(val)
+    num = val.to_i
+    return false if num.to_s != val.to_s
+    return false if num != 0 && num < CategoriesController::MIN_CATEGORIES_TOPICS
+
+    true
+  end
+
+  def error_message
+    I18n.t('site_settings.errors.invalid_integer_min', min: 5)
+  end
+end

spec/requests/categories_controller_spec.rb

@@ -481,4 +481,41 @@ describe CategoriesController do
       end
     end
   end
+
+  context '#categories_and_topics' do
+    before do
+      10.times.each { Fabricate(:topic) }
+    end
+
+    it 'works when SiteSetting.categories_topics is non-null' do
+      SiteSetting.categories_topics = 5
+
+      get '/categories_and_latest.json'
+      expect(JSON.parse(response.body)['topic_list']['topics'].size).to eq(5)
+    end
+
+    it 'works when SiteSetting.categories_topics is null' do
+      SiteSetting.categories_topics = 0
+
+      get '/categories_and_latest.json'
+      json = JSON.parse(response.body)
+      expect(json['category_list']['categories'].size).to eq(2) # 'Uncategorized' and category
+      expect(json['topic_list']['topics'].size).to eq(5)
+
+      Fabricate(:category, parent_category: category)
+
+      get '/categories_and_latest.json'
+      json = JSON.parse(response.body)
+      expect(json['category_list']['categories'].size).to eq(2)
+      expect(json['topic_list']['topics'].size).to eq(5)
+
+      Fabricate(:category)
+      Fabricate(:category)
+
+      get '/categories_and_latest.json'
+      json = JSON.parse(response.body)
+      expect(json['category_list']['categories'].size).to eq(4)
+      expect(json['topic_list']['topics'].size).to eq(6)
+    end
+  end
 end