Validating the presence only if other simple_form field equals certain input?

Question

In my simple_form a user can select a discount_type. With JQuery, the apply_on field is hidden when the DOM is loaded and only shows when the discount_type percent is selected.

Issue: I only want to validate the presence for apply_on when discount_type percent is selected, but I don't know how/where to do this?

form

<div class="col col-sm-4"><%= f.input :discount_type, collection: ['percent', 'price'] %></div>
<div class="col col-sm-4 apply_on_percentage"><%= f.input :apply_on, collection: ['reservation total', 'accommodation total'], prompt: "Choose on what to apply the discount" %></div>

<script>
  // Hide form field when DOM is loaded
  $(document).ready(function(){
    $('.apply_on_percentage').hide();
  });

  // Load form field when percentage is selected
  $(document).on("change", "#discount_discount_type", function(){
    var discount_type = $(this).val();
    if(discount_type === 'percent'){
      $('.apply_on_percentage').show();
    } else{
      $('.apply_on_percentage').hide();
    }
  });


</script>

Answer 1

As per Max' answer, I can add the following to the discount model:

validates_presence_of :apply_on, if: ->{ discount_type == 'percent' }