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

This should work

def self.table_exists?(name)
  ActiveRecord::Base.connection.tables.include?(name)
end

if table_exists?(:profile) && !Profile.column_names.include?("url")
  add_column :profile, :url, :string
end