Joachim, thanks for your explanation. I appreciate that. I was thinking if it is a good idea to write my mail. Usually this ends in a holy war which I don’t want. Comments inline.
Am 25.04.2014 um 09:02 schrieb jtuc...@objektfabrik.de: > Norbert, > > you are right, I should have given an example of what I mean. > > So here is one: > > If you serialize an object graph to, say, json and store it in a NoSQL DB, > you need to decide how "deep" you serialize your graph into one djson > document and how to build up segments of your graph that need to be > serialized separately, like for an equivalent of an 1:n relationship. > Exanple: an Order contains OrderItems, each of which reference a Product. But > the Product can be referenced from many OrderItems. So you need to "cut off" > parts of your model to serialize them separately. And you need a way to save > the "reference“. You can do that in e.g. mongo as well. You just use on ObjectId as a field value or you use a DBRef. Btw. what you describe is not 1:n but m:n. An OrderItems can have n products and a Product can be in m OrderItems. > > One of the next questions then is "what if I delete a Product?". What happens > to the OrderItems or InvoiceItems and so on? If you just delete it the collections will have a stale reference. I think there is no universal answer to that even if it seems the removal in the collection is that universal answer. If it is about integrity you need one way to make it happen. Reestablishing of integrity can happen on write time or on read time. What will happen in a SQL context? You can’t delete an object that is pointed to by a foreign key. What does it help then? Not taking your business model into account you couldn’t do anything more to find out where that reference is. That is probably the only point I wanted to make questioning your last mail. If we take that scenario you can only solve it if you take the problem one level higher (well, if you have cascading deletes you may ignore it). So these problems tend to end in the application logic. And that is what my experience tells me. Database ensured integrity isn’t much of a help in many cases. So you solve the problems in the application logic (knowing which things reference what). Being there I see no big differences to using a NoSQL database. In NoSQL those features are just not there per se. You have to model it regarding your use case. > None of these problems are unsolvable, but these problems need to be > addressed either in your persistence framework on top of a NoSQl DB or in > your application code. In Relational DBs, they've built solutions for these > problems before you and I were born ;-) I am talking of foreign keys, > referential integrities and normalization. To my knowledge, these have not > yet found their standardized counterparts in NoSQL DBs. So NoSQL can be a > good solution for many problems, but they can also be bad for many others. I’m questioning the use of each of those. As I said above I doubt there are many use cases where foreign keys are the best way to go. Btw. if you ever administrate a database and you have to recover after a crash then you might have a different view on foreign keys because they are able to make it close to impossible to load the data back. Referential integrity is either done by the database by foreign keys or in the application logic as I said above. The need for normalization is heavily use case depend. It is nothing good per se. So these are not good examples IMHO. I’m wondering you didn’t bring up the only good reason for SQL databases (for most). For me this is having atomic operations using transactions. This is the one case that can drive you nuts if try to model something with a NoSQL database to achieve it. > > I am not saying anything new here. This debate has been going on for decades > already, and much more clever people than me have made good points for both > sides over the years now. Sure. But from time to time it is good to refresh the memory. And for me you are clever enough ;) > > You also asked for examples for problems you get from using Glorp. > > One of my biggest concerns is that it can be hard to model an m:n > relationship where one or both sides of the relationship are abstract > superclasses (or even concrete ones with multiple subclasses). It gets even > harder when you want to be able to associate an object (like a Category or > Tag) to "anything". This really is hard or involves some additional coding on > top of the persistence framework. > > Glorp does miss one feature painfully: The ability to map classes with > subclasses that spread over three or more "subtables" for the subclasses. > This one is hard to explain: if you have a superclass "Vehicle" with two > subclasses "FlyingVehicle" and "DrivingVehicle", both of which have > subclasses, there is no support for a model which has tables on the > "FlyingVehicle" or "DrivingVehicle" level. You must model your DB in a way > that there are all attributes in a "Vehicle" table or in the "Car", "Train", > "Spaceship" and "Kite" tables. This really is a pity and I don't think > anybody is working on changing this any time soon. > Yep. Dealing with these cases I learned that Objects and SQL databases don’t fit together and why ORM cannot work. I mean Glorp is really good at that by providing your use case with subclasses via virtual lookups. I don’ remember how it works. It was like a descriptor with candidate classes which lead to multiple lookups. I figured that out while talking to Alan 8 years ago. I think I managed it by using a three column m:n mapping table where you have the two ids and an identifier that gives a hint which subclass/table it is to look into. Anyway even if Glorp supported sublasses lookups it was a can of worms to open. I once changed a single thing in my glorp descriptions and then the login the web site took 1,5 minutes just because all the magic in Glorp produced 432 database lookups initiate by a single button click. You just try to force a schema (OO) onto a technology (SQL) that does not support this case at all. It is an entity relationship model where the entities are flat. Inheritance is not part of the solution. So the summary would be: A lot of people making big efforts to forcing an object model into a technology that doesn’t support it just because they want to have features like integrity you can establish in other ways. This is why I don’t use SQL databases anymore until I have a use case that does benefit from it. > Does this explain what I mean? > Yes and I hope I made point also very clear :) Norbert > Joachim > > > > > > > > Am 25.04.14 08:41, schrieb Norbert Hartl: >> >> >> Am 25.04.2014 um 06:51 schrieb Joachim Tuchel <jtuc...@objektfabrik.de>: >> >>> Hi, >>> >>> We're using Glorp on VA ST for kontolino.de. It is an active project in the >>> sense of somebody is taking care of bugs. The lead developer(s) work(s) for >>> Cincom - and Cincom uses Glorp as supported component of CST. >>> Instantiations also provides Glorp with VA ST and supports it. Glorp is >>> very stable and is not moving fast, which is not a disadvantage for a >>> production system. Features are being added and bugs are fixed. >>> >>> Re: Pharoers do NoSQL: judging from the discussions on this list and >>> others, you have to be careful what you need. To me it seems many >>> developers want features from relational DBs and hammer them into their >>> applications with brute force. The first wow! soon gives room to a lot of >>> problems that have been solved in relational DBs three decades ago. >>> >> I think such a statement should be based on an example. >> >>> But it is true that o/r mapping is not always fun. It can force you to make >>> decisions about your model that look strange from the object perspective. >>> >> Agreed. And especially in the context of O/R I'm curious what will be your >> example to prove your point above. >> >> Couldn't resist, >> >> Norbert >>> I'd either go full objects (gemstone, magma) or relational for my typical >>> projects (business apps). >>> >>> Hth >>> >>> Joachim >>> >>> Clément Bera <bera.clem...@gmail.com> schrieb: >>> >>> Hello, >>> >>> I think Glorp is used with DBXTalk for relational databases by multiple >>> people. >>> >>> Usually, with Pharo, people use as a persistance layer either MongoDB with >>> the frameworks MongoTalk/Voyage. >>> >>> Gemstone is also used quite often for large scale application but Gemstone >>> is not free. >>> >>> Regards. >>> >>> >>> 2014-04-23 20:34 GMT-07:00 Pablo R. Digonzelli <pdigonze...@gmail.com>: >>> Hi all, can someone tell me if glorp is active there ar is using glorp or >>> other persistence framework in production projects? >>> >>> Tia >>> >>> Ing. Pablo Digonzelli >>> Software Solutions >>> IP-Solutiones SRL >>> Metrotec SRL >>> 25 de Mayo 521 >>> Email: pdigonze...@softsargentina.com >>> pdigonze...@gmail.com >>> Cel: 5493815982714 >>> >>> > > > -- > ----------------------------------------------------------------------- > Objektfabrik Joachim Tuchel mailto:jtuc...@objektfabrik.de > Fliederweg 1 http://www.objektfabrik.de > D-71640 Ludwigsburg http://joachimtuchel.wordpress.com > Telefon: +49 7141 56 10 86 0 Fax: +49 7141 56 10 86 1 >
