Question

How can I do a required validation a given field based on the value of another completely different field?

Answer 1

Try

attr_accessible :supplier_id, :supplier_type

validates :supplier_type, :presence  => true,
    :if => lambda{ check_supplier_exists }

def check_supplier_exists
  @supplier = Supplier.find(self.supplier_id)
  if @supplier != nil
    return true
  else 
    return false
  end
end