FIX: Delete associated channel upon category deletion

Currently when a category is deleted, if it has an associated chat
channel, the latter won’t be deleted automatically.

The fix is quite simple as we were simply missing a `dependent:
:destroy` option on the existing relation.

plugins/chat/db/post_migrate/20221117142910_delete_orphaned_channels.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+class DeleteOrphanedChannels < ActiveRecord::Migration[7.0]
+  def up
+    DB.exec(
+      "DELETE FROM chat_channels WHERE chatable_type = 'Category' AND type = 'CategoryChannel' AND NOT EXISTS (SELECT * FROM categories WHERE categories.id = chat_channels.chatable_id)",
+    )
+  end
+end

plugins/chat/lib/extensions/category_extension.rb

@@ -5,7 +5,7 @@ module Chat::CategoryExtension
 
   include Chatable
 
-  prepended { has_one :category_channel, as: :chatable }
+  prepended { has_one :category_channel, as: :chatable, dependent: :destroy }
 
   def cannot_delete_reason
     return I18n.t("category.cannot_delete.has_chat_channels") if category_channel

plugins/chat/spec/jobs/regular/auto_manage_channel_memberships_spec.rb

@@ -103,9 +103,13 @@ describe Jobs::AutoManageChannelMemberships do
     end
 
     context "when chatable doesn’t exist anymore" do
-      before do
-        channel.chatable.destroy!
-        channel.reload
+      let(:channel) do
+        Fabricate(
+          :category_channel,
+          auto_join_users: true,
+          chatable_type: "Category",
+          chatable_id: -1,
+        )
       end
 
       it "does nothing" do

plugins/chat/spec/models/category_spec.rb

@@ -8,7 +8,7 @@ RSpec.describe Category do
     let(:channel_class) { CategoryChannel }
   end
 
-  it { is_expected.to have_one(:category_channel) }
+  it { is_expected.to have_one(:category_channel).dependent(:destroy) }
 
   describe "#cannot_delete_reason" do
     subject(:reason) { category.cannot_delete_reason }

plugins/chat/spec/requests/categories_controller_spec.rb

@@ -24,6 +24,10 @@ RSpec.describe CategoriesController do
           it "deletes the category" do
             expect { destroy_category }.to change { Category.count }.by(-1)
           end
+
+          it "deletes the associated channel" do
+            expect { destroy_category }.to change { CategoryChannel.count }.by(-1)
+          end
         end
 
         context "when channel has messages" do