Ikai,
Based on your inputs I created two data classes that have a unidirectional
one-to-one relationship
Now, I have two data classes simpledata and detailscol.
simpledata contains fields A, B, C (and a Key field)
detailscol just contains field D.
simpledata imports detailscol that contains field D (and a Key field). It
also contains an accessor for the detailscol.
Code:
simpledata sdata = new simpledata(A,B,C);
sdata.setKey(null);
detailscol obj = new detailscol(D);
sdata.setD(obj);
The keys are generated by the application and then I make the data
persistent.
Now, I display just the data in simpledata and if the user clicks on a
details link I get the data stored in detailscol
To get to that data I just do
detailscol d = sdata.getDetails();
Two questions:
1) Is this the right approach?
2) If I want to get the child data using just the parent keyhow do I go
about it?
E.g, user clicks details and I use some AJAX to redirect to a different
servlet with just parent key as a parameter (since I don't access the child
object yet). I get the parent key using
KeyFactory.keyToString(sdata.getKey());
Now, that I have the parent's key should I do a getObjectbyID on the
parent data again using this and then get the child using the accessor
method or is there a direct way to construct the child key and get to the
child data.
Due to the nature of my application I would like to have the key generated
automatically (using setKey(null)).
Apologies for the confusion in advance :)
Manny
On Sat, Jan 30, 2010 at 12:16 AM, Ikai L (Google) <ika...@google.com> wrote:
> Hi Manny,
>
> A few things to first remember - App Engine's datastore is not a database,
> but a distributed key value store with additional features. Thus, we should
> be careful not to frame our thinking in terms of RDBMS schemas. For this
> reason, I like to avoid using database terminology that can confound the
> design process like "table" or "column". App Engine stores objects
> serialized ("entities") and indexes on the values. It'd be similar to an
> approach of creating a MySQL table with a String ID and a blob value,
> storing serialized Objects in the blob column, or using Memcache and storing
> JSON values.
>
> When you retrieve a single value from the key value store, we have to
> retrieve everything at once. In most scenarios, unlike SQL databases you may
> be used to, retrieving large binary or text data does not add serious
> overhead. Of course, this changes if you start storing data on the scale of
> 1mb and are retrieving it unnecessarily. How large is the data you are
> retrieving?
>
> Here's the way I would model your scenario if I was positive the
> text/binary field had a 1:1 relationship with the parent class:
>
> * on your main entity, define the properties.
> * define a new entity with a text/binary field, and encode the parent key
> information in this key such that generating the key for this child field is
> very cheap. KeyFactory.stringToKey and KeyFactory.keyToString are crucial
> here. Read more about them here:
> http://code.google.com/appengine/docs/java/javadoc/com/google/appengine/api/datastore/KeyFactory.html.
> You can call your child property "parent_id:additional_info" or whatever
> makes sense to you.
>
> Robert's solution of using a child key is basically just a variation on
> this, as parent key information is encoded in a child key.
>
> A lot of this stuff can be a bit different to get used to. I suggest
> becoming familiar with keys and how they are used in App Engine:
>
> Basic documentation about relationships:
> http://code.google.com/appengine/docs/java/datastore/relationships.html
> A more advanced article:
> http://code.google.com/appengine/articles/storage_breakdown.html
>
> On Thu, Jan 28, 2010 at 10:28 PM, Manny S <manny.m...@gmail.com> wrote:
>
>> Hi All,
>>
>> First off, thanks for your time. A quick noob question on the right way to
>> model data.
>>
>> I have a table with four columns A,B,C, D. D - the fourth is of type text
>> (contains quite a bit of data).
>>
>> I wanted to ensure that the contents of the details column 'D' is not
>> fetched during a query. A sample scenario
>> User does a search. Sees Columns A,B,C. If they need more details for that
>> particular record Click on a link that fetches D for that particular
>> record.
>>
>> So I tried to do something like - Select A, B, C from tablename.
>>
>> I found from the documentation that the GQL query returns full data
>> objects and so all queries start with SELECT *. Is this true for JDOQL on
>> the datastore as well? Does this mean everytime I query the data store its
>> going to return all columns consuming bandwidth?
>>
>> Also since I want the content of COlumn D to be fetched on subsequent user
>> action so should I instead create two tables one with
>>
>> ID_TB1, A, B, C
>>
>> and the other one with
>>
>> ID, ID_TB1, D?
>>
>> Manny
>>
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>
>
>
> --
> Ikai Lan
> Developer Programs Engineer, Google App Engine
> http://googleappengine.blogspot.com | http://twitter.com/app_engine
>
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