title: MongoDB: update only specific fields
body: <p>I am trying to update a row in a (typed) MongoDB collection with the C# driver. When handling data of that particular collection of type <code>MongoCollection&lt;User&gt;</code>, I tend to avoid retrieving sensitive data from the collection (salt, password hash, etc.)</p>
  
  <p>Now I am trying to update a <code>User</code> instance. However, I never actually retrieved sensitive data in the first place, so I guess this data would be <code>default(byte[])</code> in the retrieved model instance (as far as I can tell) before I apply modifications and submit the new data to the collection.</p>
  
  <p>Maybe I am overseeing something trivial in the MongoDB C# driver how I can use <code>MongoCollection&lt;T&gt;.Save(T item)</code> without updating specific properties such as <code>User.PasswordHash</code> or <code>User.PasswordSalt</code>? Should I retrieve the full record first, update "safe" properties there, and write it back? Or is there a fancy option to exclude certain fields from the update?</p>
  
  <p>Thanks in advance</p>