MongoDB embedded documents vs. referencing by unique ObjectIds for a system user profile

Question

I'd like to code a web app where most of the sections are dependent on the user profile (for example different to-do lists per person etc) and I'd love to use MongoDB. I was thinking of creating about 10 embedabble documents for the main profile document and keep everything related to one user inside his own document.

I don't see a clear way of using foreign keys for mongodb, the only way would be to create a field to_do_id with the type of ObjectId for example, but they would be totally unrelated internally, just happen to have the same Ids I'd have to query for.

1. Is there a limit on the number of embedded document types inside a top level document that could degrade performance?
2. How do you guys solve the issue of having a central profile document that most of the documents have to relate to in presenting a view per person?
3. Do you use semi foreign keys inside MongoDb and have fields with ObjectId types that would have some other document's unique Id instead of embedding them?

I cannot feel what approach should be taken when. Thank you very much!

Answer 1

1. There is no special limit with respect to performance. The larger the document, though, the longer it takes to transmit over the wire. The whole document is always retrieved.
2. I do it with references. You can choose between simple manual references and the database DBRef as per this page: http://www.mongodb.org/display/DOCS/Database+References
3. The link above documents how to have references in a document in a semi-foreign key way. The DBRef might be good for what you are trying to do, but the simple manual reference is very efficient.

I am not sure a general rule of thumb exists for which reference approach is best. Since I use Java or Groovy mostly, I like the fact that I get a DBRef object returned. I can check for this datatype and use that to decide how to handle the reference in a generic way.

So I tend to use a simple manual reference for references to different documents in the same collection, and a DBRef for references across collections.

I hope that helps.