validation on if condition in Laravel

How to give a validation on if condition like in my project case there are two inputs one is deal type and one is event name .so i have to give a condition like if deal type is single then i have to validate that event name otherwise no.

Here is my view :

<select name="deal_type">
      <option value="">--Select--</option>
      <option value="annual">Annual</option>
       <option value="single">Single</option>
</select>

 <select name="eventname">
      <option value="">--Select--</option>
      <option value="event1">Event1</option>
      <option value="event2">event2</option>
</select>

my validation request.php :

public function rules()
    {
         return [
                'deal_type'=>'required',
                'eventname'=>'required_if:deal_type,single'
             ]; 
}

I was trying something like above but it is not working .Any suggestions please.

via Chebli Mohamed