2009/8/28 Hector Virgen <djvir...@gmail.com>: > Thanks for the reply, Tim. So you're saying I'll need one or more mappers > for my Quiz entity depending on how much information I want to prefill the > quiz with? For example I'll have classes like: > > QuizSimpleMapper > QuizWithQuestionsMapper > > Let's say we allow the users to tag their quizzes from a set of global tags, > and they can use as many tags as they want. Would I then need more mappers? > > QuizWithTagsMapper > TagMapper > TagWithQuizzesMapper > > Then, if I need a Quiz with its questions and its tags, do I need to create > yet another mapper? > > QuizWithQuestionsAndTagMapper > > This is becoming overwhelming. I feel like this complexity could be > simplified if the entity had access to the mapper that created it, allowing > it to pull in more data in a lazy-loading fashion. But as you said, that > goes against the design pattern. > What I have been working on lately has been an "entity collection" object > which contains a mapper instance and a list of IDs. It implements the > SeekableIterator and Countable interfaces and uses lazy-loading to load the > actual entity on demand when My_Model_Entity_Collection::current() is > called. This seems to help so that objects aren't created until they're > accessed, and they can be accessed without any additional work on the > object. Any thoughts on this? > -- > Hector
I would say that the quickstart is only a pointer to how a mapper can work, once you start looking at relationships that are not simple you will need to create more and more infrastructure to handle them. The idea of a data mapper is to help create a Domain Model, this also will require components to manage object life cycle such as unit of work and identity map patterns. I would suggest reading up on domain model, Eric Evans Domain Driven Design is probably the best book to read on this. Also check out the Zend_Db_Mapper proposal, you will see from this how involved creating a data mapper is :) The good news is this proposal has been approved for development which is very good news for the framework. There is code checked into the SVN for this component too, which is worth a read through. > > > On Fri, Aug 28, 2009 at 3:32 PM, Tim Navrotskyy > <dmytro.navrots...@gmail.com> wrote: >> >> Hi, >> >> >> Hector Virgen wrote: >> > >> > The example in the Zend Framework Quick Start [1] involves only a single >> > model and a single mapper, which is fine -- the pattern works >> > beautifully >> > in >> > isolation. But in my application I have many models each with their own >> > mappers, and the models are related to each in one-to-one, one-to-many, >> > and >> > many-to-many relationships. >> > >> >> I think the data mapper pattern is not implemented right in the QuickStart >> guide which leads to series of questions like yours. Even on the >> http://martinfowler.com/eaaCatalog/dataMapper.html reference page we see: >> >> >> > A layer of Mappers (473) that moves data between objects and a database >> > while keeping them independent of each other and the mapper itself. >> > >> >> According to this statement the class Quiz may not depend on any Data >> Mapper, including the QuestionMapper. >> >> The client code (e.g. action controller) using a Data Mapper may look like >> this: >> >> //fetching questions for some quiz >> $questions = $questionMapper->findByQuiz($quiz); >> >> $questions is now a collection of objects with complete question >> informaton >> including the answer choices. >> >> The client doesn't ask the Quiz Entity to get the questions, but the >> mapper. >> The question mapper may communicate with the AnswerChoice mapper to fetch >> the choices and populate the questions with them. >> >> Implementing it this way you make your Model independent of the Mapper and >> therefore the storage type. >> >> >> Hector Virgen wrote: >> > >> > Also, since each question belongs to a quiz, should the Question object >> > contain an instance of the Quiz object it belongs to? >> > >> >> I think it's acceptable if you use this backreference somewhere. >> >> >> Hector Virgen wrote: >> > >> > If I follow this pattern, then when I "find" a single AnswerChoice >> > object, >> > the AnswerChoiceMapper would load the parent Question object, which >> > loads >> > the parent Quiz object, which loads the parent User object, etc. This >> > seems >> > to be inefficient, especially when iterating through multiple answer >> > choices. >> > >> >> With the Data Mapper layer decoupled from your model you may now implement >> another mapper for each use case which pulls exactly as many dependencies >> as >> needed. You could still use techniques you mentioned like Identity Map or >> Lazy Loading. Martin Fowler describes them very good in >> http://martinfowler.com/books.html#eaa PoEAA . >> >> Tim. >> -- >> View this message in context: >> http://www.nabble.com/Data-Mappers-and-Relational-Modelling-tp25193848p25198001.html >> Sent from the Zend Framework mailing list archive at Nabble.com. >> > > -- ---------------------------------------------------------------------- [MuTe] ----------------------------------------------------------------------
