Backfill code should be unscoped

[matrinox]

Context

If the model you're adding a column to has a default_scope (e.g. w/ soft deletion), the code to backfill won't update all rows. This can become a problem if you need to perform an unscoped query in the app and the default column is not set. This PR makes it so that all rows are updated.

Change

• Add unscoped to backfill code

Example

Given the following migration...

class AddDefaultColumn < ActiveRecord::Migration[5.0]
  def change
    add_column :users, :x, :string, default: '3'
  end
end

...it now reports the following message:

== 20190530211525 AddDefaultColumn: migrating =================================
rails aborted!
StandardError: An error has occurred, this and all later migrations canceled:

=== Dangerous operation detected #strong_migrations ===

Adding a column with a non-null default causes the entire table to be rewritten.
Instead, add the column without a default value, then change the default.

class AddDefaultColumn < ActiveRecord::Migration[5.0]
  def up
    add_column :users, :x, :string
    change_column_default :users, :x, "3"
  end

  def down
    remove_column :users, :x
  end
end

Then backfill the existing rows in the Rails console or a separate migration with disable_ddl_transaction!.

class BackfillAddDefaultColumn < ActiveRecord::Migration[5.0]
  disable_ddl_transaction!

  def change
    User.unscoped.in_batches.update_all x: "3"
  end
end

[ankane]

Looks great, thanks @matrinox 👍

[matrinox]

@ankane Thanks! ping me when you've released the version with this addition :)