How to write conditional migrations in rails?

Question

I am looking for ways to write migrations in rails that can be executed against the database many times without failing.

For instance let say I have this migration:

Answer 1

You can do something like this:

class AddUrlToProfile < ActiveRecord::Migration
  def self.up
    Profile.reset_column_information
    add_column(:profile, :url, :string) unless Profile.column_names.include?('url')

  end

  def self.down
    Profile.reset_column_information
    remove_column(:profile, :url) if Profile.column_names.include?('url')
  end
end

This will reset the column information before it begins - making sure that the Profile model has the up-to-date column information from the actual table. It will then only add the column if it doesn't exist. The same thing happens for the down function, but it only removes the column if it exists.

If you have multiple use cases for this you could factor the code out into a function and re-use that in your migrations.