title: Is MongoDB the wrong tool for this many-to-many problem?
body: <p>I'm currently designing a shop system with these entities:</p>
  
  <ul>
  <li>Account (Millions), username, email, password...</li>
  <li>Product (Millions), title, description, rating...</li>
  </ul>
  
  <p>An account can acquire a license for a product (many-to-many). I will have a page where I display all the Products licensed by a given account.</p>
  
  <p>Current concept: Product has an array of licensed Accounts, so that I can use <code>find(licensed_accounts: ObjectId("4d731fe3cedc351fa7000002"))</code>.</p>
  
  <p>I am expecting a lot of accounts. For popular products, that array might contain millions of ObjectIds (12byte * 1,000,000 = 12MB). One million will bring the document close to its current 16MB size-limit already.</p>
  
  <p>Is there a better approach to handle this? Or is MongoDB the wrong tool for that many relations?</p>