username_unique
See also
m_identity, email_unique, Forms and validation
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.