feature: Add large_user_postcode flag and index

- relevant index used by postcode collection worker will be checking for active and small
  postcodes (ie retired and large_user_postcode = false), and ordered by updated_at.
- Default to large_user_postcode: false, set to true in a rake task from the existing ONSPD
  data. Only ONSPD records will ever be large_user_postcode: true, so we only need to
  check them, but they must be set before the code to use them is added, and since this
  will timeout a migration, we have to do it as a 2-phase deploy (1: migrate, update data,
  2:  update code.)

Author: KludgeKML

db/migrate/20250211143015_update_postcodes_add_lup_flag.rb

@@ -0,0 +1,13 @@
+class UpdatePostcodesAddLupFlag < ActiveRecord::Migration[8.0]
+  def up
+    add_column :postcodes, :large_user_postcode, :boolean, default: false, null: false
+
+    add_index :postcodes, %i[retired large_user_postcode updated_at]
+  end
+
+  def down
+    remove_index :postcodes, %i[retired large_user_postcode updated_at]
+
+    remove_column :postcodes, :large_user_postcode, :boolean, default: false, null: false
+  end
+end

db/schema.rb

@@ -0,0 +1,8 @@
+desc "Update all "
+task update_lup_flag: :environment do
+  Postcode.onspd.find_in_batches(batch_size: 50) do |group|
+    group.select { |r| r.results.first["ONS"]["TYPE"] == "L" }.each do |r|
+      r.update(large_user_postcode: true)
+    end
+  end
+end

lib/tasks/update_lup_flag.rake

@@ -0,0 +1,21 @@
+require "spec_helper"
+require "rake"
+
+RSpec.describe "update_lup_flag task" do
+  let(:task) { Rake::Task["update_lup_flag"] }
+
+  context "when the update_lup_flag task is invoked" do
+    before do
+      Postcode.create(postcode: "E12AA", source: "onspd", updated_at: 3.days.ago, results: [{ "ONS" => { "TYPE" => "L" } }])
+      Postcode.create(postcode: "E13AA", source: "onspd", updated_at: 3.days.ago, results: [{ "ONS" => { "TYPE" => "S" } }])
+    end
+
+    it "sets the large_user_postcode flag true for LUP postcodes, but not for small ones" do
+      task.reenable
+      task.invoke
+
+      expect(Postcode.onspd.where(postcode: "E12AA", large_user_postcode: true).count).to eq(1)
+      expect(Postcode.onspd.where(postcode: "E13AA", large_user_postcode: false).count).to eq(1)
+    end
+  end
+end