CODEWITHSUNDEEP
pythongoogle-app-engine
specialscope
specialscope

Reputation: 4218

Google app engine, query multiple entities

I have following 2 entities.

class Photo(db.Model):
    name=db.StringProperty()
    registerdate=db.DateTimeProperty()
    iso=db.StringProperty()
    exposure=db.StringProperty()

class PhotoRatings(db.Model):
    ratings=db.IntegerProperty()

I need to do the following. Get all the photos (Photo) with iso=800 sorted by ratings (PhotoRatings). I cannot add add ratings inside Photo because ratings change all the time and I would have to write entire Photo entity every single time. This will cost me more time and money and the application will take performance hit. I read this, https://developers.google.com/appengine/articles/modeling But could not get much information from it.

EDIT: I want to avoid fetching too many items and perform the match manually. I need fast and efficient solution.

Upvotes: 0

Views: 124

Answers (2)

Peter Knego
Peter Knego

Reputation: 80340

If you look at the billing docs you'll notice that an Entity write is charged per changed number of properties.

So what you are trying to do will not reduce write cost, but will definitely increase read cost as you'll be reading double the number of entities.

Upvotes: 1

Amber
Amber

Reputation: 526603

You're trying to do relational database queries with an explicitly non-relational Datastore.

As you might imagine, this presents problems. If you want to the Datastore to sort the results for you, it has to be able to index on what you're wanting to sort. Indices cannot span multiple entity types, so you can't have an index for Photos that is ordered by PhotoRatings.

Sorry.

Consider, however - which will happen more often? Querying for this ordering of photos, or someone rating a photo? Chances are, you'll have far more views than actions, so storing the rating as part of the Photo entity might not be as big a hit as you fear.

Upvotes: 1

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