AngularJS - Optional required element

I'm attempting to use a custom directive to make a conditional required statement. The first condition I've added is 'firstInArray' to make the element required if it's the first in array of choices (necessary for a UI where you need to pick at least one item, but you could pick indefinitely many):

  .directive('variableRequired', [
    ()->
      return {
        require: 'ngModel',
        link: (scope, el, attrs, ctrl)->
          vars = attrs.variableRequired.split(',')
          condition = vars[0]
          if condition is 'firstInArray'
            item = vars[1]
            arr = vars[2]
            if scope[item] == scope[arr][0]
              $(el).removeAttr('variable-required')
              $(el).attr('required', 'required')
      }       
  ]) 

When I add scope.$apply() within my directive, the app freezes up (seems like infinite recursion).

Is there a better way to approach this than a custom directive? If not, what's wrong with my directive?