DataMapper Pattern and Transactions

[viper2843]

So I have been doing a bit of reading about the Data Mapper pattern and I mostly understand how it works. But there is one area I don't quite understand and that is how to make a transaction work with multiple mappers. For example I have two classes (Object1 and Object2) and their corresponding data mappers (Object1Mapper and Object2Mapper) and they are mapped two two database tables. When I want to perform a particular action I need to update both of those objects but I need to do that in a transaction to make sure both of those SQL statements succeed. I am not sure how to go about doing that. I should mention that this isn't for any particular project. I am just trying to understand this pattern a little better and broaden my knowledge. Does anyone know how to do this? Thanks.

[allspiritseve]

Do you make two separate calls to the mappers, or does one mapper call another when you save the first? For example, say I have a Post object with a User object as its author. I could call $postMapper->save ($post), and PostMapper could extract User and pass it to UserMapper... or, I could call $postMapper->save ($post) and $userMapper->save ($post->getAuthor()).

For the first method, you could handle transactions inside each mapper. Each mapper checks if a transaction is already started, and if not, starts one itself. If a transaction is already started, then it just goes about its business. The other option is probably a UnitOfWork... you can google that one.

[viper2843]

I understand what your saying but what if the two objects are not really related to each other? What I would like to do with mappers is similar to this pseudo code:

$dbConn->beginTransaction();
$post->save();
$someOtherObject->save();
$dbConn->commit(); 

The SQL statements from the mappers wouldn't execute until the commit. If the object contains other objects, those would also be included in the transaction. Is this sort of thing even possible with data mappers? Thanks.

[allspiritseve]

In that case you're definitely looking for UnitOfWork. Though I do wonder why you need a transaction to persist two objects that "are not really related to each other".

[viper2843]

Thank you for your replies. I think my wording of "not really related to each other" is not very good. What I mean is I want all or nothing when I persist more than one object to the database. That is why I want to wrap it in a transaction. I will read about the Unit Of Work in more detail. Thanks for your help. :-)

[matthijsA]

I have had the same problem. With nested transactions not being possible. Say

class ObjectA {
function save(){
  $dbh->start(); // start transaction
  try {
  $b = new ObjectB();
  $b->save();
  // etc
  $dbh->commit(); // commit
  } catch() {
  $dbh->rollback();
  }
}
}
class ObjectB {
  function save(){
    $dbh->start();// start transaction
    try {
      // save to db  
     $dbh->commit(); // commit
    } catch () {
      $dbh->rollback();// rollback
    }
  }
}
$a = new ObjectA();
$a->save(); 

Normally this is not possible because nested transactions are not allowed. The trick is here to let your Db handler sort out the transactions. I found a good solution in the code of EzComponents
http://ezcomponents.org/docs/tutorials/Database
you should download the framework and look at the
abstract class ezcDbHandler extends PDO

[viper2843]

Thank you for the reply. The EzComponents information proved to be informative. Thanks for your help. :-)