You will likely find it good practice to keep a createdDate (and possibly a modifiedDate) in all of your entities. Makes debugging easier, and if you're looking for this kind of pagination, there you go. Trying to abuse the key system for these purposes seems a bit hazardous.
That said, you can always generate your own String name as part of the key. This is not a trivial task given the distributed nature of appengine. Jeff On Sat, Feb 13, 2010 at 8:12 AM, Manny S <[email protected]> wrote: > Hi Tim, > Thanks again Tim. A naive question - if I use the datastore generated keys > and sort by the keys desc (for pagination) the key value of new data > inserted will be greater than that of previously entered data (if I do a > keytostring and a string compare?). > In other words does the datastore generate keys in a order or at random. I > did some experiments and I found that pagination works according to > date/time of insert if I just sort keys by descending but just wanted to > make sure. > Regards, > Manny > On Sat, Feb 13, 2010 at 1:17 PM, Tim Hoffman <[email protected]> wrote: >> >> 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 <[email protected]> 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 <[email protected]> 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 <[email protected]> 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) <[email protected]> >> > > 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 <[email protected]> >> > > > > 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) >> > > > >> <[email protected] >> > > >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 >> > > > >>> <[email protected] >> > > >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 >> > > [email protected]. >> > > > >>>> To unsubscribe from this group, send email to >> > > > >>>> >> > > > >>>> [email protected]<google-appengine%[email protected]> >> > > >> > > <google-appengine%[email protected]<google-appengine%[email protected]> >> > >> > > > >>>> . >> > > > >>>> 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 >> > > [email protected]. >> > > > >>> To unsubscribe from this group, send email to >> > > > >>> >> > > > >>> [email protected]<google-appengine%[email protected]> >> > > >> > > <google-appengine%[email protected]<google-appengine%[email protected]> >> > >> > > > >>> . >> > > > >>> 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. >> To post to this group, send email to [email protected]. >> To unsubscribe from this group, send email to >> [email protected]. >> 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 [email protected]. > To unsubscribe from this group, send email to > [email protected]. > 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 [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/google-appengine?hl=en.
