validators\UniqueValidator $targetAttribute

The name of the ActiveRecord attribute that should be used to validate the uniqueness of the current attribute value. If not set, it will use the name of the attribute currently being validated. You may use an array to validate the uniqueness of multiple columns at the same time. The array values are the attributes that will be used to validate the uniqueness, while the array keys are the attributes whose values are to be validated. If the key and the value are the same, you can just specify the value.