gistfile1.txt

@current_account = signed_in_user.accounts.find(params[:account_id])
@new_credit = @current_account.credits.build(params[:account_id])

models

class User < ActiveRecord::Base
  has_many :accounts
end

class Account < ActiveRecord::Base
  belongs_to :user
  has_many :credits
end

class Credit < ActiveRecord::Base
  belongs_to :account  
end

new credit form

<h1>Add a credit to your account<h1>
  <%= form_for [@current_account, @new_credit] do |f| %>
  
  <div class="field">
    <%= f.label :amount %><br />
    <%= f.text_field :amount %>
  </div>

  <div class="field">
    <%= f.label :description %><br />
    <%= f.text_field :description %>
  </div>

  <div class="actions"><%= f.submit %></div>

  <% end %>