The user does not know what creditcards are in your user model. You
have to have an association in the user model if you want to do
User.first.creditcards.new
has_many :creditcards
or get the users account and then check the creditcard.. Since a user
has_many accounts you would have to get an account before you could
create a creditcard.
So something like User.first.accounts.first.creditcard.new would
explain better...
Or you could use has_many through and use the accounts model as the
through...