Fetching referenced mongodb documents in another collection using Morphia

Question

I've been trying to wrap around my head around this... I have the following referenced documents in a users and groups collection.

group documents

{
"_id" : ObjectId("52eabc9914cc8d6cc1e6f723"),
"className" : "org.xxxxxx.sms.core.domain.Group",
"name" : "CE Play group",
"description" : "CE Play group",
"creationdate" : ISODate("2014-01-30T20:56:57.848Z"),
"user" : DBRef("users", ObjectId("52ea69c714ccd207329b2476"))
}
{
"_id" : ObjectId("52ea69c714ccd207329b2477"),
"className" : "org.xxxxxx.sms.core.domain.Group",
"name" : "Default",
"description" : "Default sms group",
"creationdate" : ISODate("2014-01-30T15:03:35.916Z"),
"user" : DBRef("users", ObjectId("52ea69c714ccd207329b2476"))
}

users document

{
"_id" : ObjectId("52ea69c714ccd207329b2476"),
"className" : "org.xxxxxx.core.domain.User",
"username" : "jomski2009",
"firstname" : "Jome",
"lastname" : "Akpoduado",
"email" : "jomea@example.com",
"active" : true,
"usergroups" : [
    DBRef("groups", ObjectId("52ea69c714ccd207329b2477")),
    DBRef("groups", ObjectId("52eabc9914cc8d6cc1e6f723"))
]
}

I have a Morphia Datastore singleton object which has been set up in a class to retrieve a user group and perform some manipulations on. Say I wanted to fetch the group named "Default" with a supplied username "jomski2009" , how would I achieve this in Morphia without fetching the usergroups as a list and iterating over the list of groups just to find the group I want?

Thanks.

Answer 1

DBRef is used in Mongo as a client side concept. MongoDB does not do joins so the purpose of DBRef is to hand over to the client a location from which to fetch the required object. This is handled by various client libraries in different ways.

If it is feasible for your application to do so, you might want to take a look at using Embedded Annotation instead of the Reference type. Or at the very least to include a list of usernames in your Group objects in addition to the object references. This will allow you to filter these in queries.

Also it is worthwhile looking at moving any unique identifiers like "username" to the _id field of the document as long as it is always unique. Primary key lookups are always faster.