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POHow to make composite string property in Google App Engine?
 

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  1. POHow to make composite string property in Google App Engine?
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    <p>I'm building an app where there are some events stored in the datastore, the model is (simplified) as follows: </p> <pre><code>class Event(db.Model) plan = db.ReferenceProperty(Plan) points = db.FloatProperty() date = db.DateTimeProperty() cumelativePoints = db.FloatProperty() </code></pre> <p>where Plan is the parent instance that group events to a user. My challenges are that (a) I need a consistence sort order since the sum of score of events before this event is stored in the event (b) multiple events can occur at the same time (c) the date can change (i.e., the order of the events changes) </p> <p>In <a href="https://sites.google.com/site/io/building-scalable-web-applications-with-google-app-engine" rel="nofollow">Building Scalable Web Applications with Google App Engine</a> (slide 36) a composite string property is suggested. My question is therefore how can it be implemented? I have two suggestions:</p> <ol> <li><p>Make an extra string property in the model (call it index) and a counter in plan, the new property is therefore (in psudo) <code>self.index = self.date.tostring+"|"+self.plan.counter</code>? When query order by index and when updating the date update index.</p></li> <li><p>Make an incremental counter in the model (e.g., <code>self.i</code>) and when sorting, order first by the counter (<code>i</code>) and then by date; in total <code>.order("i").order("date")</code>.</p></li> </ol> <p>Which is best or is there another solution that is better?</p> <p>Answers are highly appreciated! Sorry for the sloppy notation, bad english and all extra details from the project!</p>
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    1. PTQuestion
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    1. USTorstein I. Bø
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    1. COCould you please clarify the need for consistent sort order? The slide 36 talks about events that can occur at the same time. You are talking about inconsistent sort because the date of the event can change. It seems that if you want to go in chronological order, changing date would accurately move the order of the event up or down the timeline. However, if by "event date can change" you mean you can have multiple events of the same type, that's a different issue.
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      1. POHow to make composite string property in Google App Engine?
      1. USSologoub
    2. CO@Sologoub I agree that my language is not that clear, but what I mean is that as long as the events are not changed the sort order is unchanged. I'm not sure if it can be a problem at all, but what I understood from the video is that if you sort by date and the date is equal for multiple events the order which is returned from datastore doesn't need to stay constant (and if the sort order is reversed the returned list does not need to be reversed of each other). That's why I added another parameter in the index.
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      1. POHow to make composite string property in Google App Engine?
      1. USTorstein I. Bø
 

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