I understand the difference between embedded and referenced relationships in mongodb. By why do you need them in the first place?
MongoDB documentation has a section on this including a discussion on "why would I not want to embed this object?"
http://www.mongodb.org/display/DOCS/Schema+Design
Sometimes it's a question of the maintenance effort required to embed copies in each document, sometimes it's simply impossible to embed because of the limits on document sizes.
This question is really being asked ten times a day on IRC and various mailing lists. It's documented in depth on the MongoDB site (see above), various blog postings and talks and presentations. Basic research appreciated.
http://www.10gen.com/presentations
In addition: various SO postings deal with that:
MongoDB Schema Design - New Collection or Reference?
MongoDB Schema Design - Many small documents or fewer large documents?
As well as the 10gen resources this is a really great intro to MongoDB, the little MongoDB book is an excellent & quick read along with the interactive guides.
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