> > So now I am thinking of breaking up the WriteUps class into several
> > classes, putting all the lightweight data that I need to access
> > frequently in one class and all the heavy data that I rarely need to
> > access in another and using ancestor relationships and transactions to
> > make sure everything stays together.
> > Does this make any sense, or I am fundamentally misunderstanding how
> > app engine fetches data?
>
> Yes, that sounds like a good idea.
Where's the tradeoff point?
For example, suppose that the heavy data is accessed 90% of the time,
but all of the accesses are via a single db.get and the heavy data
instance is always in the same entity group. ("via a single db.get"
means that if the heavy data is in a separate instance and it's
needed, the same db.get is used for both the light data and the heavy
data instances).
On Oct 13, 3:49 am, "Nick Johnson (Google)" <[email protected]>
wrote:
> Hi Shailen,
>
>
>
>
>
> On Sun, Oct 11, 2009 at 4:09 PM, Shailen <[email protected]> wrote:
>
> > Consider 2 classes:
>
> > class Light(db.Model):
> > location = db.StringProperty()
>
> > class Heavy(db.Model):
> > location = db.StringProperty()
> > content = db.TextProperty()
> > image1 = db.BlobProperty()
> > image2 = db.BLobProperty()
> > image3 = db.BlobProperty()
> > ..., etc., etc. ...
>
> > I have 2 choices when getting information from the datastore: I can
> > either fetch just the keys, or I can
> > fetch the entities themselves. There isn't any way to fetch only a
> > part of an entity (say just the location property in entities of type
> > Heavy). Assuming this is correct, I have a couple of questions:
>
> > 1) If I want to access the location property in both Light and Heavy,
> > is it fair to assume that accessing Light will be speedier than
> > accessing Heavy, since Heavy has to fetch all sorts of data that I
> > don't need but that is part of the entity?
>
> Yes.
>
>
>
> > 2) Would it make sense to break down Heavy into more lightweight
> > classes for more efficient lookup?
>
> In the example above, definitely, yes.
>
>
>
> > This question is probably best clarified by discussing what's
> > happening in my actual application where I allow an Author to create
> > several WriteUps. The WriteUps have numerous properties: a heading, a
> > sub-heading, a lot of text and, optionally, several images, and a few
> > more properties. When displaying the contents of a WriteUp, I need to
> > access all these properties. But sometimes, I need to access only 1 or
> > 2 lightweight properties; for instance, on a page summarizing the
> > WriteUps done by an author, I need to only list the heading and sub-
> > heading; I do not need to access the text or Blob data. But there is
> > no way to access *just* the heading or sub-heading, right? I end up
> > fetching ALL of the entity, whether I need all of it or not. Is this
> > correct? Or does the datastore have some way of optimizing this?
>
> Entities are always fetched in their entirety.
>
>
>
>
>
>
>
> > Reading the In the documentation (in the Queries and Indexes section),
> > we are told:
>
> > # The query is not executed until results are accessed.
> > results = q.fetch(5)
> > for p in results:
> > print "%s %s, %d inches tall" % (p.first_name, p.last_name,
> > p.height)
>
> > When are results 'accessed', when fetch() is called, or when
> > p.first_name, p._last_name, etc. are used? I am assuming that when
> > fetch(5) is called, all 5 entities (assuming they exist) are fetched,
> > and if the entities contain properties other than first_name,
> > last_name or height, they are loaded up too. Is that correct?
>
> Yes.
>
>
>
> > So now I am thinking of breaking up the WriteUps class into several
> > classes, putting all the lightweight data that I need to access
> > frequently in one class and all the heavy data that I rarely need to
> > access in another and using ancestor relationships and transactions to
> > make sure everything stays together.
> > Does this make any sense, or I am fundamentally misunderstanding how
> > app engine fetches data?
>
> Yes, that sounds like a good idea.
>
> -Nick Johnson
>
>
>
> > - Shailen
>
> --
> Nick Johnson, Developer Programs Engineer, App Engine
> Google Ireland Ltd. :: Registered in Dublin, Ireland, Registration Number:
> 368047- Hide quoted text -
>
> - Show quoted text -- Hide quoted text -
>
> - Show quoted text -
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