When adding a column with a non-null default the suggested migration does not work

[Florent2]

PostgreSQL version 9.5.13, Ruby 2.3.8 and Rails 4.2.11.

When running this migration

class AddIsAdminToUsers < ActiveRecord::Migration
  def change
    add_column :users, :is_admin, :boolean, default: false, null: false
  end
end

I get this from strong_migrations:

== 20190314162335 AddIsAdminToUsers: migrating - Shard: master ================
rake 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 AddIsAdminToUsers < ActiveRecord::Migration
  def up
    add_column :users, :is_admin, :boolean, null: false
    change_column_default :users, :is_admin, false
  end

  def down
    remove_column :users, :is_admin
  end
end

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

class BackfillAddIsAdminToUsers < ActiveRecord::Migration
  disable_ddl_transaction!

  def change
    User.find_in_batches do |records|
      User.where(id: records.map(&:id)).update_all is_admin: false
    end
  end
end

But I create those suggested migrations and run them, the first one fails with the error:

== 20190314162335 AddIsAdminToUsers: migrating - Shard: master ================
-- add_column(:users, :is_admin, :boolean, {:null=>false})
rake aborted!
StandardError: An error has occurred, this and all later migrations canceled:

PG::NotNullViolation: ERROR:  column "is_admin" contains null values
: ALTER TABLE "users" ADD "is_admin" boolean NOT NULL

This is because of null: false in the suggested line

add_column :users, :is_admin, :boolean, null: false

Should instead this line be the following instead?

add_column :users, :is_admin, :boolean, null: true

and an additional third migration should be suggested to set the NOT NULL constraint?

change_column_null :users, :is_admin, false

[ankane]

Hey @Florent2, thanks for reporting 👍 Yeah, the null option should be removed from the first migration if it's false (since true is the default) and a third migration would be needed to change the null value. Feel free to submit a PR, or I'll get around to it when I have some time.