Using the Data Mapper Pattern, Should the Entities (Domain Objects) know about the Mapper?

Question

I'm working with Doctrine2 for the first time, but I think this question is generic enough to not be dependent on a specific ORM.

Should the entities in a Data Mapper pattern be aware - and use - the Mapper?

I have a few specific examples, but they all seem to boil down to the same general question.

If I'm dealing with data from an external source - for example a User has many Messages - and the external source simply provides the latest few entities (like an RSS feed), how can $user->addMessage($message) check for duplicates unless it either is aware of the Mapper, or it 'searches' through the collection (seems like an inefficient thing to do).

Of course a Controller or Transaction Script could check for duplicates before adding the message to the user - but that doesn't seem quite right, and would lead to code duplication.

If I have a large collection - again a User with many Messages - how can the User entity provide limiting and pagination for the collection without actually proxying a Mapper call?

Again, the Controller or Transaction Script or whatever is using the Entity could use the Mapper directly to retrieve a collection of the User's Messages limited by count, date range, or other factors - but that too would lead to code duplication.

Is the answer using Repositories and making the Entity aware of them? (At least for Doctrine2, and whatever analogous concept is used by other ORMs.) At that point the Entity is still relatively decoupled from the Mapper.

Answer 1

Rule #1: Keep your domain model simple and straightforward.

First, don't prematurely optimize something because you think it may be inefficient. Build your domain so that the objects and syntax flow correctly. Keep the interfaces clean: $user->addMessage($message) is clean, precise and unambiguous. Underneath the hood you can utilize any number of patterns/techniques to ensure that integrity is maintained (caching, lookups, etc). You can utilize Services to orchestrate (complex) object dependencies, probably overkill for this but here is a basic sample/idea.

class User
{
  public function addMessage(Message $message)
  {
     // One solution, loop through all messages first, throw error if already exists
     $this->messages[] $message;
  }
  public function getMessage()
  {
     return $this->messages;
  }
}
class MessageService
{
  public function addUserMessage(User $user, Message $message)
  {
     // Ensure unique message for user
     // One solution is loop through $user->getMessages() here and make sure unique
     // This is more or less the only path to adding a message, so ensure its integrity here before proceeding 
     // There could also be ACL checks placed here as well
     // You could also create functions that provide checks to determine whether certain criteria are met/unmet before proceeding
     if ($this->doesUserHaveMessage($user,$message)) {
       throw Exception...
     }
     $user->addMessage($message);
  }
  // Note, this may not be the correct place for this function to "live"
  public function doesUserHaveMessage(User $user, Message $message)
  {
     // Do a database lookup here
     return ($user->hasMessage($message) ? true
  }
}
class MessageRepository
{
  public function find(/* criteria */)
  {
     // Use caching here
     return $message;
  }
}

class MessageFactory
{
   public function createMessage($data)
   {
     //
     $message = new Message();
     // setters
     return $message;
   }
}

// Application code
$user = $userRepository->find(/* lookup criteria */);
$message = $messageFactory->create(/* data */);
// Could wrap in try/catch
$messageService->sendUserMessage($user,$message);

Been working with Doctrine2 as well. Your domain entity objects are just that objects...they should not have any idea of where they came from, the domain model just manages them and passes them around to the various functions that manage and manipulate them.

Looking back over, I'm not sure that I completely answered your question. However, I don't think that the entities themselves should have any access to the mappers. Create Services/Repositories/Whatever to operate on the objects and utilize the appropriate techniques in those functions...

Don't overengineer it from the onset either. Keep your domain focused on its goal and refactor when performance is actually an issue.

Answer 2

IMO, an Entity should be oblivious of where it came from, who created it and how to populate its related Entities. In the ORM I use (my own) I am able to define joins between two tables and limiting its results by specifying (in C#) :

SearchCriteria sc = new SearchCriteria();
sc.AddSort("Message.CREATED_DATE","DESC");
sc.MaxRows = 10;
results = Mapper.Read(sc, new User(new Message());

That will result in a join which is limited to 10 items, ordered by date create of message. The Message items will be added to each User. If I write:

results = Mapper.Read(sc, new  Message(new User());

the join is reversed.

So, it is possible to make Entities completely unaware of the mapper.