title: MongoDB embedded documents vs. referencing by unique ObjectIds for a system user profile
body: <p>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.</p>
  
  <p>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.</p>
  
  <ol>
  <li>Is there a limit on the number of embedded document types inside a top level document that could degrade performance?</li>
  <li>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?</li>
  <li>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?</li>
  </ol>
  
  <p>I cannot feel what approach should be taken when. Thank you very much!</p>