Query on ReferenceProperty in Google App Engine

I have two entity kinds and one referring to other like:

class Entity1(db.Expando):
     prop1=db.StringProperty()
     prop2=db.StringProperty()

class Entity2(db.Expando):
     prop3=db.ReferenceProperty(Entity1)
     prop4=db.StringProperty()

Can I write a query like:

q=Entity2.all().filter("prop3.prop1 =",somevalue)

Here prop3 has a reference and will be referring to some entity of kind Entity1 and I want to know all those entities of kind Entity2 which开发者_StackOverflow社区 refer to those entities of Entity1 that have prop1 as somevalue.

In my example I first fetched the key for the referenceproperty to use as a filter for the query. Then you can do the filtering based on the key in a the same amounts of queries you found keys for.

An example:

The models:

class Order(db.Model):
    waiter = db.ReferenceProperty(Waiter,required=True)
    date=db.DateTimeProperty(required=True)
    delivered=db.BooleanProperty(default=False)

class Waiter(db.Model):
    firstname=db.StringProperty(required=True)
    lastname=db.StringProperty(required=True)
    uuid=db.StringProperty(required=True)

The web request function:

def get(self):
        waiteruuid=self.request.get("waiter")
        q=Waiter.all()
        q.filter('uuid =', waiteruuid)
        waiterkey=q.get()
        result={}
        result['orders']=[]
        q=Order.all()
        if waiteruuid:
            q.filter('waiter =', waiterkey)
        orders=q.run()

Google Datastore doesn't support joins. You can fetch all the entites of Entity2 and does some manipulation to achieve what you have said.Somewhat similar to what @Mani suggested. But you can do it like

entities2 = Entity2.all()
for entity2 in entities2:
   Entity1= entity.prop3.get()
    if Entity1.prop1== somevalue:
       #Do your processing here

Define Entity 2 as:

class Entity2(db.Expando):
     entity_1_ref = db.ReferenceProperty(Entity1, collection_name = "set_of_entity_2_elements")
     prop4=db.StringProperty()

This defines a collection name which can be operated from the other side of reference. (Entity1 in this case). I have taken the liberty to rename prop3 as something more appropriate.

Now you can do q = entity_1_object.set_of_entity_2_elements (a new property for all your Entity1 objects) which will give you the results you want.

For more information, read this article indepth: http://code.google.com/appengine/articles/modeling.html

Update: Sorry, I got it wrong. The above suggestion doesnt get only those elements with entity_1_object.prop1 == somevalue

You can still get it in a round about way as follows:

for obj in q:
    if ( obj.prop1 == somevalue):
        # Do your processing here

or

for obj in q:
    if ( obj.prop1 != somevalue):
        # Delete this object from the list 'q'

But obviously this is not the best way. Lets wait for a better answer!