The primaryKey for elasticsearch documents is the
_id field by default. This field is not part of the
ActiveRecord attributes so you should never add
_id to the list of [[attributes()|attributes]].
You may override this method to define the primary key name when you have defined
path mapping
for the
_id field so that it is part of the
_source and thus part of the [[attributes()|attributes]].
Note that elasticsearch only supports _one_ attribute to be the primary key. However to match the signature
of the [[\yii\db\ActiveRecordInterface|ActiveRecordInterface]] this methods returns an array instead of a
single string.
