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

Wrapping my migration in a conditional worked for me. Rails 4.X

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

    def self.down
      remove_column :profile, :url
    end
  end
end