Entity groups, ReferenceProperty or key as a string

Question

After building a few application on the gae platform I usually use some relationship between different models in the datastore in basically every application. And often I find my self the need to see what record is of the same parent (like matching all entry with same parent)

From the beginning I used the db.ReferenceProperty to get my relations going, like:

class Foo(db.Model):
    name = db.StringProperty()

class Bar(db.Model):
    name = db.StringProperty()
    parentFoo = db.ReferanceProperty(Foo)

fooKey = someFooKeyFromSomePlace
bars = Bar.all()
for bar in bar:
    if bar.parentFoo.key() == fooKey:
        // do stuff

But lately I've abandoned this approch since the bar.parentFoo.key() makes a sub query to fetch Foo each time. The approach I now use is to store each Foo key as a string on Bar.parentFoo and this way I can string compare this with someFooKeyFromSomePlace and get rid of all the subquery overhead.

Now I've started to look at Entity groups and wondering if this is even a better way to go? I can't really figure out how to use them.

And as for the two approaches above I'm wondering is there any downsides to using them? Could using stored key string comeback and bit me in the * * *. And last but not least is there a faster way to do this?

Answer 1

Tip:

replace...

bar.parentFoo.key() == fooKey

with...

Bar.parentFoo.get_value_for_datastore(bar) == fooKey

To avoid the extra lookup and just fetch the key from the ReferenceProperty

See Property Class