How to cache referenced entities in Morphia? - mongodb

Question

I'm using Morphia for MongoDB with Stripes Framework.
Let us assume I have two entities, Car (which describes a specific car, say some particular 1984 Honda Accord) and CarType (which specifies all Honda Accords of that kind):
The most natural way to model this seems:
#Entity
class Car {
#Id private String id; // VIN
private Date purchaseDate;
private Color color;
#Reference private CarType type;
// ..
}
#Entity
class CarType {
#Id private String id;
private String manufacturerId;
private float engineDisplacement;
// ..
}
This works, but is inefficient, as CarType is looked up from DB every time a Car is loaded. I would like to cache car types in memory, as they change rarely. Persistence frameworks like GORM and Hibernate would allow that out of the box, but I'm not sure how to do it under Morphia (there is a feature request raised for that).
I'd like to keep the reference to CarType, as just storing a String carTypeId would complicate the views and everything else too much.
So I thought I could do something like this:
#Entity
class Car {
#Id private String id; // VIN
private Date purchaseDate;
private Color color;
private String typeId;
#Transient private CarType type;
#Transient private CarService service = new CarServiceImpl();
public void setTypeId() {
this.typeId = typeId;
updateTypeReference();
}
#PostLoad void postLoad() {
updateTypeReference();
}
private void updateTypeReference() {
type = service.findTypeById(typeId);
}
// ..
}
class CarServiceImpl implements CarService {
#CacheResult CarType findCarTypeId(String typeId) {
datastore.get(CarType.class, typeId);
}
// ..
}
Which works and does what I want, but:
Does seem like a hack
I'd to inject the service instead using Guice, but cannot figure out how, although I have overall dependency injection working in Stripes ActionBeans.
So I'd like to either:
Learn how to inject (preferably, Guice) services into Morphia entities
or
Learn how to otherwise properly do caching for referenced entities in Morphia
or
If all else fails, switch to some other MongoDB POJO mapping approach which supports caching. But I really like Morphia so I'd rather not.

Answer 1

Another common approach would be to embed the CarType in each Car. That way you would only have to fetch a single entity.
Trade-offs:
You'll need an update logic for all duplicated CarTypes. Since you said that they hardly change, this should be fine performance-wise.
Duplicated data requires additional disk-space and the working set in RAM gets bigger as well.
You'll need to evaluate how this works out for your data, but data duplication to make reads faster is quite a common approach...

Answer 2

Since I didn't think of a better solution I am doing a #PostLoad event handler which gets the datastore class from a static variable, and can then look up the Referenced entity.
That seems like a hack and requires the datastore service to be thread-safe, but it works for me.