Input validation exclusive or

Question

I have a model called Answer.

This model has two possible relations. A Question or a Product. But, Answer should only ever have one relation. Either Question or Product.

There is a form to create answers.

This form has three inputs. two of them are <select> inputs. The other is a text input, called name.

I want my validation to only allow one to be filled out.

My current validation:

$validator = Validator::make(
    Input::all(),
    array('name' => array('required'))
);

$validator->sometimes('product', array('required', 'numeric'), function ($input) {
    return !is_numeric($input->question);
});

$validator->sometimes('question', array('required', 'numeric'), function ($input) {
    return !is_numeric($input->product);
});

Requires at least one of the selected to be filled out, but will also allow two.

So, my question is: How can I change my validation to only allow one of the selects to be filled out. But one of them must always be filled out.

Select 1:

<select name="question" class="form-control">
    <option>None</option>
    <option value="1" selected="selected">Question 1</option>
    <option value="2">Question 2</option>
</select>

Select 2:

<select name="product" class="form-control">
    <option>None</option>
    <option value="2" selected="selected">Product 1</option>
    <option value="3">Product 2</option>
</select>

Answer 1

@Razor's custom XOR validation rule is pretty nice, but there's another route if you don't want to create a custom rule. You can add values to Input representing your constraints, using Input::merge, then use those for validation:

Input::merge(array(
    'hasBoth'    => Input::has('product')  && Input::has('question'),
    'hasNeither' => !Input::has('product') && !Input::has('question')
));

$validator = Validator::make(
    Input::all(), array(
        'name'       => 'required',
        'hasNeither' => 'size:0',
        'hasBoth'    => 'size:0',
    ), array(
        'hasNeither.size' => 'A question or a product is required.',
        'hasBoth.size'    => 'You cannot choose both a question and a product.'         
    )
);

You should still change the empty value in your form to <option value=''>None</option>.

Unlike using an XOR comparison, this method allows you to return separate error messages for no values vs. two values without any further error checking.