PHP OOP: Avoid Singleton/Static Methods in Domain Model Pattern
- by sunwukung
I understand the importance of Dependency Injection and its role in Unit testing, which is why the following issue is giving me pause:
One area where I struggle not to use the Singleton is the Identity Map/Unit of Work pattern (Which keeps tabs on Domain Object state).
//Not actual code, but it should demonstrate the point
class Monitor{//singleton construction omitted for brevity
static $members = array();//keeps record of all objects
static $dirty = array();//keeps record of all modified objects
static $clean = array();//keeps record of all clean objects
}
class Mapper{//queries database, maps values to object fields
public function find($id){
if(isset(Monitor::members[$id]){
return Monitor::members[$id];
}
$values = $this->selectStmt($id);
//field mapping process omitted for brevity
$Object = new Object($values);
Monitor::new[$id]=$Object
return $Object;
}
$User = $UserMapper->find(1);//domain object is registered in Id Map
$User->changePropertyX();//object is marked "dirty" in UoW
// at this point, I can save by passing the Domain Object back to the Mapper
$UserMapper->save($User);//object is marked clean in UoW
//but a nicer API would be something like this
$User->save();
//but if I want to do this - it has to make a call to the mapper/db somehow
$User->getBlogPosts();
//or else have to generate specific collection/object graphing methods in the mapper
$UserPosts = $UserMapper->getBlogPosts();
$User->setPosts($UserPosts);
Any advice on how you might handle this situation?
I would be loathe to pass/generate instances of the mapper/database access into the Domain Object itself to satisfy DI - At the same time, avoiding that results in lots of calls within the Domain Object to external static methods.
Although I guess if I want "save" to be part of its behaviour then a facility to do so is required in its construction. Perhaps it's a problem with responsibility, the Domain Object shouldn't be burdened with saving. It's just quite a neat feature from the Active Record pattern - it would be nice to implement it in some way.
