framework/db/BaseActiveRecord.php
public function offsetUnset($offset)
    {
        if (property_exists($this, $offset)) {
            $this->$offset = null;
        } else {
            unset($this->$offset);
        }
    }
public function offsetUnset($offset)
    {
        if (property_exists($this, $offset)) {
            $this->$offset = null;
        } else {
            unset($this->$offset);
        }
    }
Sets the element value at the specified offset to null.
This method is required by the SPL interface [[\ArrayAccess]].
It is implicitly called when you use something like `unset($model[$offset])`.
@param mixed $offset the offset to unset element