title: Check If Field Has Been Modified
body: <p>I'm working on a user system, and I want to use the <code>beforeValidate</code> hook to hash the user's password with bcrypt. However, if the password is not changed, I want it to skip hashing the password. I know it's possible in MongoDB/Mongoose, but I haven't stumbled across anything comparable to Mongoose's <code>isModified</code> function.</p>
  
  <p>Is there anything that I can use to check if it's modified? Or would be setting the password via an <code>InstanceMethod</code> be the only way to do that?</p>