I have a basic ActiveRecord model in which i have two fields that i would like to validate. The requirement is that at least one of the fields must have a value. Both can have values, but at least one needs a value.

How do i express this with 

```
validates_presence_of 

```

statements? For example:

```
validates_presence_of :main_file
validates_presence_of :alt_file

```

i don't want an error to be generated if only one of them is empty, only if both are empty.