Add assets:get_id_by_legacy_url_path rake task

It seems unnecessarily laborious to have to run rails console
commands (https://docs.publishing.service.gov.uk/manual/manage-assets.html#get-an-assets-id)
in order to know what to pass to a rake task
(https://docs.publishing.service.gov.uk/manual/manage-assets.html#redirect-an-asset).

It should just be a rake task.

lib/tasks/assets.rake

@@ -25,4 +25,11 @@ namespace :assets do
     asset = WhitehallAsset.find_by!(legacy_url_path: args.fetch(:legacy_url_path))
     Rake::Task["assets:redirect"].invoke(asset.id, args.fetch(:redirect_url))
   end
+
+  desc "Get a Whitehall asset's ID by its legacy_url_path, e.g. /government/uploads/system/uploads/attachment_data/file/1234/document.pdf"
+  task :get_id_by_legacy_url_path, %i[legacy_url_path] => :environment do |_t, args|
+    legacy_url_path = args.fetch(:legacy_url_path)
+    asset = WhitehallAsset.find_by!(legacy_url_path:)
+    puts "Asset ID for #{legacy_url_path} is #{asset.id}."
+  end
 end

spec/lib/tasks/assets_spec.rb

@@ -0,0 +1,32 @@
+require "rails_helper"
+require "rake"
+
+RSpec.describe "assets.rake" do
+  before do
+    AssetManager::Application.load_tasks if Rake::Task.tasks.empty?
+  end
+
+  describe "get_id_by_legacy_url_path" do
+    before do
+      task.reenable # without this, calling `invoke` does nothing after first test
+    end
+
+    let(:task) { Rake::Task["assets:get_id_by_legacy_url_path"] }
+
+    it "returns ID of asset by its legacy URL path" do
+      id = "abc123def456ghi789"
+      legacy_url_path = "/government/uploads/system/uploads/attachment_data/file/1234/document.pdf"
+      FactoryBot.create(:whitehall_asset, id:, legacy_url_path:)
+
+      expected_output = <<~OUTPUT
+        Asset ID for #{legacy_url_path} is #{id}.
+      OUTPUT
+
+      expect { task.invoke(legacy_url_path) }.to output(expected_output).to_stdout
+    end
+
+    it "raises exception if no asset found" do
+      expect { task.invoke("foo") }.to raise_error(Mongoid::Errors::DocumentNotFound)
+    end
+  end
+end