Try
<div class="form-group">
<label class="col-sm-2 control-label'">Select permissions for this role</label>
<div class="col-sm-6">
<%= f.select(:permissions, options_for_select(Xaaron::Permission.pluck(:permission),
:selected => @role.permissions), {}, {:multiple => true, :class => 'form-control'}) %>
</div>
</div>