Hi Manny Understood.
You know you can let the system generate the entity identifiers for you, and pagination will just work (by __key__) any way. So you don't have to the key_name or id at all if you don't care what the key is. Rgds T On Feb 13, 1:00 am, Manny S <manny.m...@gmail.com> wrote: > Thanks a bunch Tim for your inputs, > > My rationale for adding the date to the appstore generated key is to make > pagination easier. I would do pagination on the key and not add a separate > column for that. (Pagination by date alone will also not solve my problem > since it can have duplicates and hence couple it with the key) > > I understand fetching by keys is much faster. Though I don't see a scenario > where I would have to do that now I would like to architect my app where > that would be possible. However, I do not have anything unique in my record > with which I can set the key. It just contains city name, locality details > and a series of other fields all of which could have duplicates. Any ideas > as to how I can generate unique ids from these or any general pointers > towards generating unique Ids from data where the data itself does not have > a unique field? > > Manny > > On Thu, Feb 11, 2010 at 4:49 PM, Tim Hoffman <zutes...@gmail.com> wrote: > > 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> > > <google-appengine%2bunsubscr...@googlegroups.com<google-appengine%252bunsubscr...@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> > > <google-appengine%2bunsubscr...@googlegroups.com<google-appengine%252bunsubscr...@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 > > ... > > read more » -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. 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