Applying bootstrap has-error styling to a select2 element

With Bootstrap 3.3 and Select2 4.0.6 the error styling may be applied as follows:

css:

.has-error {border:1px solid rgb(185, 74, 72) !important;}

jquery/javascript:

$('#YourSelect2ControlID').next().find('.select2-selection').addClass('has-error');

For Select2 controls the object containing the border styling is not the <select> element, it is one of the nested <span> elements following the <select> element. That particular span contains the .select2-selection class.

The solution on the page you have posted, is for an older version of select2. For the latest version, use this CSS code here:

.has-error .select2-selection {
    border-color: rgb(185, 74, 72) !important;
}

Now with this code and using the has-error class, you can get the proper reddish error color:

<div class="form-group has-error">
 <select name="description_id" class="select2">
  <option>One</option>
  <option>Two</option>
  <option>Three</option>
  <option>Four</option>
 </select>
</div>

jsFiddle example: https://jsfiddle.net/k9phxgnd/1/