Validates Presence of Child Object

Hi Folks,

Is there a validation in rails that works on child objects that are
declared as associations.

eg: I have an Order class and a LineItem class, where Order has_many
:line_items. Is there a validation that would allow me to ensure that
an order is only valid when it has at least on line item?


Sure, validates_size_of :line_items, :minimum => 1