FIX: Order categories in SQL for Categories#search (#26810)

Otherwise, the results don't make sense if the number of categories is
more than the limit provided.

app/controllers/categories_controller.rb

@@ -412,6 +412,13 @@ class CategoriesController < ApplicationController
         )
         .joins("LEFT JOIN topics t on t.id = categories.topic_id")
         .select("categories.*, t.slug topic_slug")
+        .order(
+          "starts_with(lower(categories.name), #{ActiveRecord::Base.connection.quote(term)}) DESC",
+          "categories.parent_category_id IS NULL DESC",
+          "categories.id IS NOT DISTINCT FROM #{ActiveRecord::Base.connection.quote(prioritized_category_id)} DESC",
+          "categories.parent_category_id IS NOT DISTINCT FROM #{ActiveRecord::Base.connection.quote(prioritized_category_id)} DESC",
+          "categories.id ASC",
+        )
         .limit(limit)
         .offset((page - 1) * limit)
 
@@ -421,19 +428,6 @@ class CategoriesController < ApplicationController
 
     Category.preload_user_fields!(guardian, categories)
 
-    # Prioritize categories that start with the term, then top-level
-    # categories, then subcategories
-    categories =
-      categories.to_a.sort_by do |category|
-        [
-          category.name.downcase.starts_with?(term) ? 0 : 1,
-          category.parent_category_id.blank? ? 0 : 1,
-          category.id == prioritized_category_id ? 0 : 1,
-          category.parent_category_id == prioritized_category_id ? 0 : 1,
-          category.id,
-        ]
-      end
-
     response = {
       categories_count: categories_count,
       categories: serialize_data(categories, SiteCategorySerializer, scope: guardian),

spec/requests/categories_controller_spec.rb

@@ -1408,6 +1408,18 @@ RSpec.describe CategoriesController do
           [category4.id, category2.id, category3.id, category1.id],
         )
       end
+
+      it "returns categories in the correct order when the limit is lower than the total number of categories" do
+        categories =
+          4.times.flat_map do |i|
+            get "/categories/search.json", params: { term: "ordered", page: i + 1, limit: 1 }
+            response.parsed_body["categories"]
+          end
+
+        expect(categories.map { |c| c["id"] }).to eq(
+          [category4.id, category2.id, category3.id, category1.id],
+        )
+      end
     end
 
     it "returns user fields" do