title: It's not possible to lock a mongodb document. What if I need to?
body: <p>I know that I can't lock a single mongodb document, in fact there is no way to lock a collection either.</p>
  
  <p>However, I've got this scenario, where I think I need some way to prevent more than one thread (or process, it's not important) from modifying a document. Here's my scenario.</p>
  
  <p>I have a collection that contains object of type A. I have some code that retrieve a document of type A, add an element in an array that is a property of the document (<code>a.arr.add(new Thing()</code>) and then save back the document to mongodb. This code is parallel, multiple threads in my applications can do theses operations and for now there is no way to prevent to threads from doing theses operations in parallel on the same document. This is bad because one of the threads could overwrite the works of the other.</p>
  
  <p>I do use the repository pattern to abstract the access to the mongodb collection, so I only have CRUDs operations at my disposition.</p>
  
  <p>Now that I think about it, maybe it's a limitation of the repository pattern and not a limitation of mongodb that is causing me troubles. Anyway, how can I make this code "thread safe"? I guess there's a well known solution to this problem, but being new to mongodb and the repository pattern, I don't immediately sees it.</p>
  
  <p>Thanks</p>