How does YIIrules verify that the user does not exist?

Native rules can verify whether the user name is repeated [& #039; username & #039;, & #039; unique & #039;, & #039; targetClass & #039; & amp; gt; & #039; appmodelsStudent & #039;, & #039; message & #039; & amp; gt; & #039; usernamerepeat & #039;], in this way, I mean, when you log on to the table, check whether the table exists... native rules can verify whether the user name is repeated
['Username', 'unique', 'targetclass' => '\ app \ models \ Student', 'message' => 'username repeat'],

In this way, I mean, when a user logs on to the table, check whether there is a user name. If the user name does not exist, the user name does not exist. Is there such a rule, if not, do I have to write ajax on my own?

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Native rules can verify whether the user name is repeated
['Username', 'unique', 'targetclass' => '\ app \ models \ Student', 'message' => 'username repeat'],

In this way, I mean, when a user logs on to the table, check whether there is a user name. If the user name does not exist, the user name does not exist. Is there such a rule, if not, do I have to write ajax on my own?

I am a genius. rules has exist rules, so it's OK.
[['Username'], 'exist', 'targetclass' => '\ app \ models \ Student', 'message' => 'username not exist'],