UniqueValidator checks if the value being validated is unique in the table column specified by
the ActiveRecord class [[targetClass]] and the attribute [[targetAttribute]].
The following are examples of validation rules using this validator:
php
a1 needs to be unique
['a1', 'unique']
a1 needs to be unique, but column a2 will be used to check the uniqueness of the a1 value
['a1', 'unique', 'targetAttribute' => 'a2']
a1 and a2 need to be unique together, and they both will receive error message
[['a1', 'a2'], 'unique', 'targetAttribute' => ['a1', 'a2']]
a1 and a2 need to be unique together, only a1 will receive error message
['a1', 'unique', 'targetAttribute' => ['a1', 'a2']]
a1 needs to be unique by checking the uniqueness of both a2 and a3 (using a1 value)
['a1', 'unique', 'targetAttribute' => ['a2', 'a1' => 'a3']]