title: Document Databases: Redundant data, references, etc. (MongoDB specifically)
body: <p>It seems like I run into lots of situations where the appropriate way to build my data is to split it into two documents.  Let's say it was for a chain of stores and you were saving which stores each customer had visited.  Stores and Customers need to be independent pieces of data because they interact with plenty of other things, but we do need to relate them.</p>
  
  <p>So the easy answer is to store the user's Id in the store document, or the store's Id in the user's document.  Often times though, you want to access 1-2 other pieces of data for display purposes because Id's aren't useful.  Like maybe the customer name, or the store name.</p>
  
  <ol>
  <li>Do you typically store a duplicate of the entire document?  Or just store the pieces of data you need?  Maybe depends on the size of the doc vs how much of it you need.</li>
  <li>How do you handle the fact that you have duplicate data?  Do you go hunt down data when it changes?  Update the data at some interval when it's loaded?  Only duplicate when you can afford stale data?</li>
  </ol>
  
  <p>Would appreciate your input and/or links to any kind of 'best practices' or at least well-reasoned discussion of these topics.</p>