Htykuut

TL;DR - Using an explicit database connection in a migration breaks reversibility.

I have a simple migration, adding a column to a specific database connection: (My app has several databases, for good reasons)

class AddFlavorToBikes < ActiveRecord::Migration

  def change

    VehicleBase.connection.tap do |db|

      db.add_column :bikes, :flavor, :string

    end

  end

end

This works great:

 % be rake db:migrate

== 20181120215337 AddFlavorToBikes: migrating =================================

== 20181120215337 AddFlavorToBikes: migrated (0.0060s) ========================

However, it fails to rollback:

% be rake db:rollback

== 20181120215337 AddFlavorToBikes: reverting =================================

rake aborted!

An error has occurred, this and all later migrations canceled:



PG::DuplicateColumn: ERROR:  column "flavor" of relation "bikes" already exists

: ALTER TABLE "bikes" ADD "flavor" character varying/Users/david/rider-gate/db/migrate/20181120215337_add_flavor_to_bikes.rb:5:in `block in change'

/Users/david/rider-gate/db/migrate/20181120215337_add_flavor_to_bikes.rb:3:in `tap'

/Users/david/rider-gate/db/migrate/20181120215337_add_flavor_to_bikes.rb:3:in `change'

I don't understand this. The rollback of db.add_column should remove the column. So why am I getting an error that the field I'm trying to remove already exists? Of course it exists, that's why I'm trying to remove it.

I searched the Interwebs for a solution, or even anyone with the same problem, and did not find any leads.

I tried using an explicit variable instead of .tap, but got the same error:

class AddFlavorToBikes < ActiveRecord::Migration

  def change

    db = VehicleBase.connection

    db.add_column :bikes, :flavor, :string

  end

end

The closest I've been able to discern, ActiveRecord::Migration loses its ability to detect if it is migrating up or down on any but the default ActiveRecord::Base connection.

So, it tries to migrate add_column UP, even though it is in a rollback and should be migrating it DOWN. Hence, it is trying to add the column a second time, instead of reversing the add_column into a remove_column.

This is on Rails 4.2.7 and Ruby 2.1.9

How can I make this migration reversible?

I found a reasonable solution by splitting change into up and down methods:

class AddFlavorToBikes < ActiveRecord::Migration

  def up

    VehicleBase.connection.tap do |db|

      db.add_column :bikes, :flavor, :string

    end

  end



  def down

    VehicleBase.connection.tap do |db|

      db.remove_column :bikes, :flavor

    end

  end

end

While not as graceful or DRY as a reversible migration, this allows db:migrate and db:rollback to work succesfully.