How to map relations between two huge collections in AppEngine datastore

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Ralph Zajac

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Mar 7, 2012, 6:59:08 AM3/7/12

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I think the best way to explain my problem will be an example what I'm trying to do.

Lets say I have Message and City entities. Each message is posted by a User about a City. Now Users can add as many cities as they want to their favorites.

I have to design the data model in the way that User sees Messages only about the Cities they have in favorites (sorted by date).

First I thought about using reference property pointing to City in Message model. But then I would have to use queries like this:

db.GqlQuery("SELECT * FROM Message WHERE city IN (1, 2, 3, 4, ....)

This will obviously not scale and break if User has more then 30 Cities in favorites.