Getting random entities from the DataStore

Sometimes it amazes me how easy it is to do some things using App Engine/Django, and how some simple things are a goddamn science. This one is somewhere in the middle. Getting a random entity from the Datastore. This is how you do it!

# Setting up the model class myModel(BaseModel): random = db.FloatProperty(required=True) # Creating an entity myModelInstance = myModel(random = random.random()) myModelInstance.save() # Querying the datastore gql = "SELECT * FROM myModel WHERE random >= :random ORDER BY random ASC LIMIT 1" randomSantaRelation = GqlQuery(gql, random=random.random()).get() if randomSantaRelation is None: gql = "SELECT * FROM myModel WHERE random <= :random ORDER BY random DESC LIMIT 1" randomSantaRelation = GqlQuery(gql, random=random.random()).get()

See what happens here? We assign a random float to the entity on it's creation - they will look something like this in the datastore:

0.216565955485

Then, when doing a query, we simply create a new random value and query the Datastore for a single entity that is larger than that random value. In the off chance that this query returns no results, we'll have a backup query that runs the whole thing in reverse. This can happen if you have very few entities, and none of them have a random value in the upper ranges:

0.628912291991
0.416565323566
0.216565955485
0.118278328322
0.013212121212

If the random value you send into the GqlQuery above is 0.898912291991, it will return no rows. Therefore, the backup query is needed. The system will, of course, rely less and less on the backup query as more entities are added.