Can Rails Migrations be used to convert data?

Question

I'm trying to convert a column in my Rails app, for arguments sake let's pretend I'm trying to change the age column in my users table to a string representation rather than an int.

In my migration I have this;

def.self up     add_column :users, :age_text, :string      users = User.find(:all)      users.each do |u|        u.age_text = convert_to_text(u.age)        u.save     end end  def self.convert_to_text(number)    #code here to convert 1 to 'one' etc end 

But it doesn't seem to be working, is what I'm attempting here even possible with migrations?

Answer 1

What you're trying to do is possible, and I would say the correct thing to do.

You need, though, to reload the column info for the model classes you're updating in the migration, so that Rails knows about the new columns. Try this:

def.self up     add_column :users, :age_text, :string      User.reset_column_information       users = User.find(:all)      users.each do |u|        u.age_text = convert_to_text(u.age)        u.save     end end 

On a separate note, please note that if your table is large, doing updates one by one will take a looong time.. Be careful with that.

Answer 2

Since I'm new here I can't comment on the above so I'll add my own answer.

GENERALLY manipulating data in migrations is a BAD idea. Migrations with direct model access can get stuck if the model logic changes.

Imagine in your second migration you've added a new column. You want to seed that column with new data.

Let's also say a few weeks later you add a new validation to the model - a validation that operates on a field that does not yet exist in your second migration. if you ever were to construct the database from migration 0, you'd have some problems.

I strongly suggest using migrations to alter columns and other means to manage database data, especially when moving to production.