What are the recommended ways to handle references to embedded documents in MongoDB? - mongodb

Question

I'm new to MongoDB. Here's my problem: a user can have multiple avatars, but one and only one is active. Here's what a user document looks like for the moment:
{
"_id": ObjectId("515c99f7e4d8094a87e13757"),
"avatars": [{
"url": "http://example.com/img/photo1.jpg",
"width": 50,
"height": 50,
}, {
"active": true,
"url": "http://example.com/img/photo2.jpg",
"width": 50,
"height": 50,
}]
}
This solution is simple enough, but there are a few things I don't like:
changing the active avatar means updating two embedded documents
I'm not sure it will behave nicely in case of concurrent access (read_avatar+change_active_avatar or change_active+change_active) (will it?)
looking for the active avatar requires a sequential search
Another solution would be this:
{
"_id": ObjectId("515c99f7e4d8094a87e13757"),
"active_avatar_id": 2,
"avatars": [{
"_id": 1,
"url": "http://example.com/img/photo1.jpg",
"width": 50,
"height": 50,
}, {
"_id": 2,
"url": "http://example.com/img/photo2.jpg",
"width": 50,
"height": 50,
}]
}
This fixes problems 1&2, but not problem 3. And it adds an extra _id field in every embedded document. Plus when I insert a new avatar I now need to know what's the next _id to use (unless I use an objectId, but then it's a 12 bytes id for just a few hundred avatars (max).
So yet another solution could be this:
{
"_id": ObjectId("515c99f7e4d8094a87e13757"),
"active_avatar": {
"url": "http://example.com/img/photo2.jpg",
"width": 50,
"height": 50,
},
"extra_avatars": [{
"url": "http://example.com/img/photo1.jpg",
"width": 50,
"height": 50,
}]
}
Not much better than the first solution (it just fixes problem #3, that's it, and it's uglier).
All these solutions work, but I'm looking for "the right way" to do it. Any ideas? And what if I allow users to have multiple active avatars (whatever that would mean)?
Thanks.

Answer 1

Have you considered the document inside a document model?
{
"_id" : ObjectId("515c99f7e4d8094a87e13758"),
"active_avatar_id" : 2,
"avatars" : {
"1" : {
"url" : "http://example.com/img/photo1.jpg",
"width" : 50,
"height" : 50
},
"2" : {
"url" : "http://example.com/img/photo2.jpg",
"width" : 50,
"height" : 50
}
}
}