我有一个表单，有几个输入、一些文本、一些单选/下拉字段。所有字段都是必填项。对于文本输入，我收到一条很好的消息，上面写着"；填写此字段"；，但对于Radio和Dropdown字段，该消息不会显示。如何显示单选字段和下拉字段的错误消息？以下是我的代码：
<form action="" method="post" class="form" role="form">
<div class="form-group required"><label class="form-control-label" for="name">Name:</label>
<input class="form-control" id="name" name="name" required="" type="text" value="">
</div>
<!-- ^ this works -->
<div class="form-group required"><label class="form-control-label" for="gender">Gender</label>
<div class="form-check">
<input class="form-check-input" id="preference-0" name="preference" type="radio" value="male">
<label class="form-check-label" for="preference-0">Male</label>
</div>
<div class="form-check">
<input class="form-check-input" id="preference-1" name="preference" type="radio" value="female">
<label class="form-check-label" for="preference-1">Female</label>
</div>
<div class="form-check">
<input class="form-check-input" id="preference-2" name="preference" type="radio" value="other">
<label class="form-check-label" for="preference-2">Other</label>
</div>
</div>
<!-- ^ this does not work -->
<div class="form-group required"><label class="form-control-label" for="choice">
Please choose "A" or "B"</label>
<select class="form-control" id="which_day" name="which_day" required="">
<option value="a">A</option>
<option value="b">B</option><
</select>
</div>
<!-- ^ neither does this... -->
<input class="btn btn-primary btn-md" id="submit" name="submit" type="submit" value="Next">
</form>
我现在已经想好了，至少你需要把"必需"；在一个收音机/一个下拉字段中，如下所示：
<div class="form-check">
<input class="form-check-input" id="preference-0" name="preference" type="radio" value="male" required>
<label class="form-check-label" for="preference-0">Male</label>
</div>
对于选择字段，这里有一个答案，说明了如何做到这一点。