Make ActiveStorage update migration compatible with install migration

[etiennebarrie]

If you install ActiveStorage in rails 6.1, then use rails app:update, the service name migration handles that case but not the variant records one, which fails trying to re-create the table.

Summary

I updated an app to Rails 6.1, but forgot to run rails app:update immediately. Then later I installed ActiveStorage, and finally came back to run the update task.
When I saw the update migrations being created in the app, I assumed they would fail but was pleasantly surprised that the service name passed by checking for the presence of the column. So I felt maybe we'd want to do the same to the other migration. We can also assume users will understand and just remove the migrations.

Repro steps:

$ rails _6.1.1_ new my_app && cd my_app
$ rails active_storage:install && rails db:migrate
$ rails app:update && rails db:migrate
# fails on CreateActiveStorageVariantRecords

Other Information

It works perfectly in the regular case:

$ rails _6.0.3.4_ new my_app && cd my_app
$ rails active_storage:install && rails db:migrate
$ $EDITOR Gemfile && bundle update # update to 6.1
$ yes | rails app:update && rails db:migrate

[rafaelfranca]

Let's change to

Suggested change

	create_table :active_storage_variant_records do |t|
	create_table :active_storage_variant_records, if_not_exists: true do |t|

[etiennebarrie]

Oh right I had forgotten about this 👍
Also works with as a reversible migration directly.

[rails-bot]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.
Thank you for your contributions.

[etiennebarrie]

This is still good to go!

[rails-bot]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.
Thank you for your contributions.