Hi Manny Do you really want to do that for a key. One if the big advantages of creating your own keys is being able to explicitly get entities by key (1 or more with db.get(list_of_keys) which is much quicker than a gql or filter. Making your keys include dates mean you will be unlikely to guess/know what the keys are in advance.
This of course may not be useful for what you are doing, but worth keeping in mind. Rgds T On Feb 11, 2:12 pm, Manny S <manny.m...@gmail.com> wrote: > Hi Ikai, > > I did read the documentation and now I have my data structures in place. One > thing I wanted to do and that was not clear from my previous post was to > append a app generated string (not unique) as a prefix to a datastore > generated key. For instance, I want to generate a key that has the date (of > record creation) as a prefix to the datastore generated unique key. Is > there a way to do this? I do not want my application to generate unique Ids. > > From reading through the literature so far, I am guessing that will not be > possible since the datastore keys are generated only at the time when the > objects are being made persistent. > > Manny > > On Wed, Feb 10, 2010 at 3:30 AM, Ikai L (Google) <ika...@google.com> wrote: > > > Have you read our documentation on KeyFactory? > > >http://code.google.com/appengine/docs/java/datastore/relationships.html > > > <http://code.google.com/appengine/docs/java/datastore/relationships.html>I'd > > try to understand what's going on there. It sounds like you're doing it the > > right way, but it's up to you to benchmark and find the best approach for > > what works for you. The usage characteristics of your application should > > determine the way your store your data. > > > On Wed, Feb 3, 2010 at 3:42 AM, Manny S <manny.m...@gmail.com> wrote: > > >> 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/appengi.... > >>> 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 > > >>>> -- > >>>> You received this message because you are subscribed to the Google > >>>> Groups "Google App Engine" group. > >>>> To post to this group, send email to google-appeng...@googlegroups.com. > >>>> To unsubscribe from this group, send email to > >>>> google-appengine+unsubscr...@googlegroups.com<google-appengine%2bunsubscr...@googlegroups.com> > >>>> . > >>>> For more options, visit this group at > >>>>http://groups.google.com/group/google-appengine?hl=en. > > >>> -- > >>> Ikai Lan > >>> Developer Programs Engineer, Google App Engine > >>>http://googleappengine.blogspot.com|http://twitter.com/app_engine > > >>> -- > >>> You received this message because you are subscribed to the Google > >>> Groups "Google App Engine" group. > >>> To post to this group, send email to google-appeng...@googlegroups.com. > >>> To unsubscribe from this group, send email to > >>> google-appengine+unsubscr...@googlegroups.com<google-appengine%2bunsubscr...@googlegroups.com> > >>> . > >>> For more options, visit this group at > >>>http://groups.google.com/group/google-appengine?hl=en. > > >> -- > >> You received this message because you are subscribed to the Google Groups > >> "Google App Engine" group. > >> To post to this group, send email to google-appeng...@googlegroups.com. > >> To unsubscribe from this group, send email to > >> google-appengine+unsubscr...@googlegroups.com<google-appengine%2bunsubscr...@googlegroups.com> > >> . > >> For more options, visit this group at > >>http://groups.google.com/group/google-appengine?hl=en. > > > -- > > Ikai Lan > > Developer Programs Engineer, Google App Engine > >http://googleappengine.blogspot.com|http://twitter.com/app_engine > > > -- > > You received this message because you are subscribed to the Google Groups > > "Google App Engine" group. > > To post to this group, send email to google-appeng...@googlegroups.com. > > To unsubscribe from this group, send email to > > google-appengine+unsubscr...@googlegroups.com<google-appengine%2bunsubscr...@googlegroups.com> > > . > > For more options, visit this group at > >http://groups.google.com/group/google-appengine?hl=en. -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. 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