I am able to properly pass a string variable to the gqlquery through parameter substitution, here's the code i've tried to use;

```
user_name = self.request.get('username') #retrieved from UI
p = models.UserDetails.all().filter('user_name = ', user_name).fetch(1)

```

I don't get any results and the query fails silently. But when I hard code the query like this ,

```
p = models.UserDetails.all().filter('user_name = ', "peter rice").fetch(1)

```

I get my expected resultset. I think I am passing the variable `user_name` in a wrong way, Please help me in getting my piece of code right.