Illuminate\Database\Eloquent\Model 中delete方法源码:
 
public function delete()
{
 if (is_null($this->getKeyName())) {
  throw new Exception('No primary key defined on model.');
 }
 if (! $this->exists) {
  return;
 }
 if ($this->fireModelEvent('deleting') === false) {
  return false;
 }
 $this->touchOwners();
 $this->performDeleteOnModel();
 $this->fireModelEvent('deleted', false);
 return true;
}
protected function performDeleteOnModel()
{
 $this->setKeysForSaveQuery($this->newModelQuery())
 ->delete();
 $this->exists = false;
}

protected function performDeleteOnModel()
{
 if ($this->forceDeleting) {
  $this->exists = false;
  return $this->newModelQuery()->where(
    $this->getKeyName(), $this->getKey()
  )->forceDelete();
 }
 return $this->runSoftDelete();
}
 
protected function runSoftDelete()
{
 $query = $this->newModelQuery()
      ->where($this->getKeyName(), $this->getKey());
 $time = $this->freshTimestamp();
 $columns = [$this->getDeletedAtColumn() => $this->fromDateTime($time)];
 $this->{$this->getDeletedAtColumn()} = $time;
 if ($this->timestamps && ! is_null($this->getUpdatedAtColumn())) {
  $this->{$this->getUpdatedAtColumn()} = $time;
  $columns[$this->getUpdatedAtColumn()] = $this->fromDateTime($time);
 }
 $query->update($columns);
}