How to validate numericality and inclusion while still allowing attribute to be nil in some cases?

Question

In a Rails app I have several integer attributes on a model.

A user should be able to create a record and leave these attributes blank.

Or, if the user ente

Answer 1

should be simply:

validates_numericality_of :a, :only_integer => true, :message => "can only be whole number.", :allow_nil => true

same for the second validation