title: Best way to check if value already exists in datastore
body: <p>I have made a registration form for a blog I made using Google Datastore, and now I an trying to make it better.</p>
  
  <p>First thing I seen that can be changed is the way that web application looks for the username in the data store, to see is the username already used.</p>
  
  <p>I did it this way:</p>
  
  <pre><code>db.GqlQuery("SELECT * FROM UserData where username = :1", username)
  </code></pre>
  
  <p>Can you please refer me to better solution, which will be faster.</p>
  
  <p>Thank you. </p>