Rails - converting data during migrations

Question

I'm splitting out some data from a model into a new one and creating an association between them.

I've added the new_model_id to OldModel, created NewModel with address as a string attribute, and have the following migrations:

class RemoveAddressFromOldModel < ActiveRecord::Migration
  def up
    OldModel.where("address IS NOT NULL AND address != ''").each do |i|
      j = NewModel.create(address: i.address)
      i.new_model_id = j.id
      i.save
    end
    remove_column :old_models, :address
  end

  def down
    add_column :old_models, :address, :string
  end
end

While the NewModel.create is working perfectly and getting the right data, the i.save doesn't seem to be working, as old_models.new_model_id isn't being populated.

What am I missing with this?

score 2 · Answer 1 · answered Sep 22 '12 at 04:54

This sounds like validations failing to me, though it could also be a before_* callback that's preventing the save. I can't speculate further without the OldModel code.

Call save when you expect it to fail and you plan to check the return value; call save! when you expect it to succeed want an exception instead. I'd use save! here because I would want the migration to fail if the save fails.

Also: your migration copies data from OldModel to NewModel in up, but doesn't copy it the other way in down. In this configuration, rake db:migrate:redo would succeed but silently discard information. If this information is important to you, I'd recommend either a) making the migration bidirectional by copying the data back or b) removing down so Rails knows it can't un-do.

Rails - converting data during migrations

1 Answers1