Selecting based on __key__ (a unique identifier) in google appengine [Java]

I have

public class QuantityType {
    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
    private Key key;

    @Persistent
    private String type;
}

I am trying to setup a query to get the right QuantityType by it's key

gql = "select * from QuantityType where __key__='aght52oobW1hIHTWVzc2FnZRiyAQw'";

But its not working because

BadFilterError: BadFilterError: invalid filter: key filter value must be a Key; received aght52oobW1hIHTWVzc2FnZRiyAQw (a str).

I have also tried to use

gql = "select * from QuantityType where __key__=='" + KeyFactory.stringT开发者_如何学PythonoKey(qTypeKey)+"'";

but it's not working..

How can I get a specific object from my datastore by it's key?

First, you should never construct a GQL string by hand - this leads to injection vulnerabilities. Instead, declare and pass in parameters, as documented here.

To retrieve an entity by key, though, you don't need to do a query at all: Use getObjectById, as documented here. This is significantly faster than using a query.