Introduce a recurring job to delete unused tags

ref: https://37s.fizzy.37signals.com/collections/693169850/cards/999009072

app/jobs/delete_unused_tags_job.rb

@@ -0,0 +1,9 @@
+class DeleteUnusedTagsJob < ApplicationJob
+  def perform
+    ApplicationRecord.with_each_tenant do |tenant|
+      Tag.unused.find_each do |tag|
+        tag.destroy!
+      end
+    end
+  end
+end

app/models/tag.rb

@@ -8,6 +8,7 @@ class Tag < ApplicationRecord
   normalizes :title, with: -> { it.downcase }
 
   scope :alphabetically, -> { order("lower(title)") }
+  scope :unused, -> { left_outer_joins(:taggings).where(taggings: { id: nil }) }
 
   def hashtag
     "#" + title

config/recurring.yml

@@ -11,3 +11,6 @@ production:
   sqlite_backups:
     class: SQLiteBackupsJob
     schedule: every day at 05:00
+  delete_unused_tags:
+    class: DeleteUnusedTagsJob
+    schedule: every day at 04:00

test/jobs/delete_unused_tags_job_test.rb

@@ -0,0 +1,13 @@
+require "test_helper"
+
+class DeleteUnusedTagsJobTest < ActiveJob::TestCase
+  test "deletes tags that are not used by any cards" do
+    unused = Tag.create!(title: "unused")
+
+    assert_changes -> { Tag.count }, -1 do
+      DeleteUnusedTagsJob.perform_now
+    end
+
+    assert_not Tag.exists?(unused.id), "Unused tag should be deleted"
+  end
+end

test/models/tag_test.rb

@@ -4,4 +4,18 @@ class TagTest < ActiveSupport::TestCase
   test "downcase title" do
     assert_equal "a tag", Tag.create!(title: "A TAG").title
   end
+
+  test ".unused returns tags not associated with any cards" do
+    unused = Tag.create!(title: "unused")
+
+    unused_tags = Tag.unused
+
+    assert_includes unused_tags, unused
+    assert_not_includes unused_tags, tags(:web)
+    assert_not_includes unused_tags, tags(:mobile)
+  end
+
+  test ".unused returns empty relation if all tags are used" do
+    assert_empty Tag.unused
+  end
 end