_form.html.js

<script type="text/javascript">
  function loadCapabilityFromBudgetAllocation(event, budget_allocation) {
    if (budget_allocation) {

      var capabilities = budget_allocation.capabilities;

      if (capabilities.length == 1) {
        $(this).closest('.nested-extra_credit_moviment_types').find('.capability').val(capabilities[0].description);
        $(this).closest('.nested-extra_credit_moviment_types').find('.capability').next('input').val(capabilities[0].id);
        $(this).closest('.nested-extra_credit_moviment_types').find('.capability').attr('disabled','disabled');
      } else {
        disableOrEnableCapability($(this), true);
        $(this).closest('.nested-extra_credit_moviment_types').find('.capability').data('modal-url', '<%= modal_capabilities_path %>?by_budget_allocation_id='+ budget_allocation.id);
      }
    } else {
      disableOrEnableCapability($(this), true);
    }
  }

  function cleanCapability(budget) {
    budget.closest('.nested-extra_credit_moviment_types').find('.capability').val('');
    budget.closest('.nested-extra_credit_moviment_types').find('.capability').next('input').val('');
  }

  function disableOrEnableCapability(budget, clean) {
    if (clean) {
      cleanCapability(budget);
    }

    if (budget.val() == '') {
      budget.closest('.nested-extra_credit_moviment_types').find('.capability').attr('disabled','disabled');
    } else {
      budget.closest('.nested-extra_credit_moviment_types').find('.capability').removeAttr('disabled');
    }
  }

  $('.nested-extra_credit_moviment_types').on('change', '.budget-allocation', loadCapabilityFromBudgetAllocation);

  $.each($('.budget-allocation'), function(i, v) {
    disableOrEnableCapability($(v));
  });
</script>