username_unique
Check if an entered username is unique, by looking in the m_identity table for the given username:
<input type="text" id="username" name="username" value="" />
{% validate id="username" type={username_unique} %}
Optionally, an id parameter can be given to the
validator to skip that particular id when doing the uniqueness
check. This is useful when you are displaying a form in which the user
is editing his own user name.