code: public function setDepth() {
      $self = $this;
  
      $this->getConnection()->transaction(function() use ($self) {
        $self->reload();
  
        $level = $self->getLevel();
  
        $self->newNestedSetQuery()->where($self->getKeyName(), '=', $self->getKey())->update(array($self->getDepthColumnName() => $level));
        $self->setAttribute($self->getDepthColumnName(), $level);
      });
  
      return $this;
    }
docs: Sets the depth attribute
  
  @return \Baum\Node