title: how to make case insensitive query using variable?
body: <p>I already have a user with a username <strong>Kevin</strong> in my database. I want prevent new users to pick <strong>Kevin</strong>, <strong>kevin</strong>, <strong>KeViN</strong>, etc. as their username when creating an account. How do I modify my query to check if such user exists ignoring case-sensitivity?</p>
  
  <pre><code>def validate_username(self, username):
      if db.users.find_one({"username":username}):
          raise ValidationError("username is already taken")
  </code></pre>